Reasoning Requirements for Arbitral Awards in Spain

Under Spanish arbitration law, reasoned awards are the rule, not the exception. Article 37(4) of the Spanish Arbitration Act (Ley 60/2003, de 23 de diciembre, de Arbitraje — “Spanish Arbitration Act” or “SAA”) expressly provides that an arbitral award “shall always be reasoned”, except where the award records terms agreed by the parties.

However, Spanish constitutional jurisprudence has made an important distinction: the obligation to give reasons is statutory rather than equivalent to the constitutional duty imposed on Spanish courts under Article 24 of the Constitution. Consequently, Spanish courts exercising annulment jurisdiction generally cannot use an alleged deficiency in reasoning as an opportunity to reconsider the merits, evidence, legal interpretation, or factual conclusions of the tribunal.

The modern Spanish position is principally shaped by STC 46/2020, STC 17/2021, STC 65/2021, STC 50/2022, STC 146/2024 and STC 49/2026.

1. Statutory foundation: Article 37(4) of the Spanish Arbitration Act

Article 37 regulates the form and contents of the award. Article 37(4) establishes the basic rule:

Every award must be reasoned, unless it is an award incorporating the terms agreed by the parties.

Thus, a Spanish-seated tribunal ordinarily cannot issue a purely dispositive award without reasons.

The statutory requirement has several consequences.

A. The award must disclose the tribunal's reasoning

The award should enable the parties to understand why the tribunal reached its conclusion.

This does not mean that the tribunal must reproduce every submission, every evidentiary item, or every argument advanced by counsel. The reasoning must instead make the decisional path intelligible.

A good award should therefore normally identify:

1. the relevant issues;
2. the tribunal's findings of fact;
3. the applicable contractual/legal framework;
4. the principal arguments of the parties;
5. the tribunal's analysis;
6. the reasons for accepting or rejecting the decisive arguments; and
7. the connection between the reasoning and the dispositive portion.

B. Consent awards are different

Article 37(4) expressly excludes awards made in accordance with an agreement between the parties under Article 36. Where the tribunal merely records a settlement in the form of an award, the ordinary substantive reasoning requirement does not operate in the same manner.

C. Reasoning requirements can be strengthened by party agreement

This is particularly important in Spanish arbitration.

The statutory minimum is not necessarily the maximum requirement. Parties may establish procedural requirements concerning the form and content of the award. The Constitutional Court has emphasized that arbitration is based on party autonomy, and the parties may influence the standard applicable to the award's reasoning.

Accordingly, a clause or institutional rule requiring a more detailed explanation may become relevant when determining whether the tribunal complied with its obligations.

2. The crucial distinction between judicial and arbitral reasoning

The most important principle in Spanish law is that the reasoning of an arbitral award is not governed by precisely the same constitutional standard as the reasoning of a judicial judgment.

Spanish courts are subject to Article 24.1 of the Constitution concerning effective judicial protection, while the requirement that arbitrators provide reasons derives principally from Article 37(4) SAA.

This distinction was decisively articulated by the Spanish Constitutional Court in STC 17/2021 and STC 65/2021.

The Constitutional Court held that the obligation to reason an award is a statutory requirement and does not itself constitute a separate fundamental right equivalent to the right to a reasoned judicial decision.

This has major practical consequences.

A court is not entitled to ask:

“Would I have reasoned this differently?”

Instead, the essential question is:

“Does the award contain reasons from which the tribunal's decision can be understood?”

That is a substantially narrower form of judicial scrutiny.

3. The tribunal does not have to answer every argument

A common misconception is that Article 37(4) requires an arbitral tribunal to address every argument, every document and every piece of evidence.

That is not the Spanish constitutional standard.

In STC 17/2021, the Constitutional Court expressly rejected the proposition that Article 37(4) requires the arbitrator to decide every argument presented by the parties, identify every piece of evidence relied upon, or explain why one legal rule was preferred over another in exhaustive detail.

The essential requirement is that the award contain reasons permitting the parties to understand the basis of the decision.

Therefore:

No requirement of exhaustive reasoning

does not mean

no requirement of reasoning at all.

A completely unexplained dispositive conclusion may still constitute a failure to comply with Article 37(4).

4. STC 17/2021 — foundational decision

STC 17/2021, 15 February 2021

This is one of the most important authorities on reasoning requirements in Spanish arbitration.

The Constitutional Court emphasized that an award does not require an exhaustive and detailed discussion of every possible aspect of the dispute. What is required is reasoning that makes the essential criteria underlying the decision identifiable.

The Court also stressed the relationship between reasoning and arbitral autonomy.

The judicial annulment court cannot transform itself into an appellate court examining whether the arbitrator:

• interpreted the contract correctly;
• selected the best legal rule;
• correctly assessed witness evidence;
• correctly evaluated expert evidence; or
• reached the conclusion that the judge would have reached.

The annulment court's function remains external and limited.

Significance

STC 17/2021 therefore establishes a basic proposition:

The existence of intelligible reasons is required; judicial agreement with those reasons is not.

This distinction is central to Spanish arbitration jurisprudence.

5. STC 65/2021 — incorrect reasoning does not necessarily mean insufficient reasoning

STC 65/2021, 15 March 2021

This case involved a challenge to an arbitral award in which the Madrid High Court of Justice had treated alleged deficiencies in the tribunal's reasoning as a matter of public policy.

The Constitutional Court rejected that approach.

It held that Article 37(4) does not require:

• an answer to every argument;
• identification of every item of evidence;
• detailed explanation of why one piece of evidence was preferred;
• exhaustive explanation of every legal proposition.

Most importantly, the reasons contained in the award may be considered incorrect by the reviewing court without automatically becoming a failure of motivation.

The Court stated that where the arbitrator has actually reasoned and argued the decision, the requirement of Article 37(4) can be satisfied even though a court might have assessed the evidence or law differently.

Practical rule from STC 65/2021

There is a critical difference between:

absence of reasons

and

reasons that a court considers wrong.

The first can potentially constitute a defect.

The second generally cannot justify annulment merely because the court disagrees.

6. STC 46/2020 — judicial intervention must remain limited

STC 46/2020, 15 June 2020

Although STC 46/2020 was not exclusively concerned with Article 37(4), it is foundational for understanding judicial review of arbitral reasoning.

The Constitutional Court warned against expanding “public policy” (orden público) so broadly that Spanish courts effectively become appellate courts over arbitration.

The Court emphasized that arbitration is based upon party autonomy and that judicial intervention must remain limited.

The significance for reasoning is substantial.

If every allegedly inadequate explanation could be converted into a public-policy violation, parties could use an annulment action to obtain a de facto appeal.

STC 46/2020 rejects that approach.

Consequence

An annulment court cannot say:

“The arbitrator's reasoning is wrong, therefore the award violates public policy.”

The court must identify a genuine legally recognized ground for annulment.

This principle became even more explicit in subsequent cases.

7. STC 55/2021 — reinforcement of the restricted judicial role

STC 55/2021, 15 March 2021

STC 55/2021 reinforced the Constitutional Court's general doctrine concerning the limited role of the courts in annulment proceedings.

Although the case was principally concerned with the judicial handling of an annulment proceeding following an agreement between the parties, it forms part of the jurisprudential sequence following STC 46/2020 and preceding STC 65/2021.

Its broader importance lies in reinforcing the principle that the constitutional requirements applicable to the judicial decision on annulment must not be confused with the substantive reasoning requirements applicable to the arbitral award.

The distinction can be expressed as follows:

8. STC 50/2022 — erroneous arbitral reasoning is not automatically public policy

STC 50/2022, 4 April 2022

STC 50/2022 is particularly useful because it concerned an award that had been annulled on the basis of allegedly erroneous reasoning concerning a potential criminal-law issue.

The Constitutional Court reiterated that courts exercising annulment jurisdiction cannot review the merits merely because they disagree with the tribunal's reasoning.

The Court again confirmed:

• annulment is not an appeal;
• courts cannot reassess evidence;
• courts cannot substitute their own legal reasoning for that of the tribunal;
• the reasoning requirement under Article 37(4) is not equivalent to the judicial duty of reasoning under Article 24;
• inadequacy of arbitral reasoning does not automatically become a public-policy violation.

Importance

STC 50/2022 therefore protects the finality and autonomy of arbitration.

The court's role is essentially to determine whether the award falls within one of the statutory grounds for annulment, rather than deciding whether the tribunal reached the best or most convincing answer.

9. STC 146/2024 — modern formulation of the test

STC 146/2024, 2 December 2024

STC 146/2024 provides one of the clearest modern statements of Spanish law.

The Constitutional Court reiterated that an arbitral award does not require exhaustive or highly detailed reasoning concerning every argument or perspective raised by the parties. Instead, the award must contain the elements and reasons necessary to identify the legal or equitable criteria underlying the decision, and those reasons must not be arbitrary.

The Court further confirmed that Article 37(4):

• requires motivation;
• does not prescribe a particular length;
• does not require an exhaustive structure;
• does not necessarily require discussion of every item of evidence;
• does not necessarily require the tribunal to respond individually to every submission.

Key test

The award should provide a sufficiently cognizable and rational decisional path.

Thus, the test is essentially:

Can a reasonable reader understand why the tribunal reached the dispositive result?

If yes, the statutory requirement will ordinarily be satisfied.

10. STC 49/2026 — latest significant confirmation

STC 49/2026, 23 June 2026

This is particularly important because it is the most recent authority in the present jurisprudential line.

The Constitutional Court reaffirmed that Article 37(4) merely requires that the award be reasoned. It does not impose upon arbitrators the same constitutional standard applicable to judicial decisions, nor does it require a particular length, exhaustive analysis, or predetermined structure.

The Court further emphasized that the award need not expressly refer to every item of evidence or provide an individualized explanation of the tribunal's assessment of each item, provided that the award contains reasons from which the decision-making criterion can be reconstructed.

This is a particularly strong confirmation of the modern Spanish approach.

11. The present Spanish test

Taking the six principal cases together, the Spanish position can be summarized in a five-part test.

Test 1 — Is there reasoning?

The first question is whether the award actually contains reasons.

A completely unexplained conclusion is problematic because Article 37(4) expressly requires motivation.

Test 2 — Can the decision be understood?

The reasons should permit the reader to understand the essential path from:

facts → applicable rules/principles → analysis → conclusion.

Test 3 — Is the reasoning non-arbitrary?

The tribunal has considerable freedom, but its reasoning cannot be merely fictitious, internally meaningless, or wholly disconnected from the issues submitted to it.

Test 4 — Must every argument be addressed?

No.

The tribunal does not ordinarily need to respond separately to every submission or item of evidence.

Test 5 — Can the court substitute its own reasoning?

No, ordinarily not.

The annulment court cannot replace the tribunal's interpretation of the contract, assessment of evidence, or application of ordinary law merely because it considers another solution preferable.

12. Reasoning in arbitration de derecho

In a legal arbitration (arbitraje de derecho), the tribunal ordinarily decides according to applicable law.

The award should therefore ordinarily explain:

1. which substantive law governs;
2. which contractual provisions are relevant;
3. which statutory rules apply;
4. how the tribunal interprets those provisions;
5. how the facts satisfy—or fail to satisfy—the legal requirements; and
6. why the resulting relief is justified.

However, even in legal arbitration, the tribunal does not need to provide an academic legal treatise.

For example, suppose a tribunal finds:

• the claimant established breach;
• the contractual termination clause applies;
• the respondent failed to establish the contractual excuse;
• the claimant proved specified losses; and
• the contractual limitation clause restricts recovery.

A concise but coherent explanation of those conclusions may satisfy Article 37(4).

The court cannot annul merely because it believes the tribunal misinterpreted the termination clause, absent an applicable statutory ground for annulment.

13. Reasoning in arbitration de equidad

The position is even more distinctive in arbitration de equidad.

In equity arbitration, the tribunal may decide according to considerations of fairness and justice rather than mechanically applying substantive legal rules.

STC 17/2021 and STC 146/2024 recognize that the reasoning requirement is consequently somewhat more flexible.

But equity does not mean absence of reasons.

The tribunal should still explain why, considering the circumstances, it considers one solution fairer or more equitable than another.

Thus:

Equity arbitration relaxes the legal methodology, not the obligation to explain the decision.

The reasoning may be non-legal, but it must still disclose the basis on which the tribunal selected the particular equitable solution.

14. Can poor reasoning lead to annulment?

Yes—but this needs to be carefully qualified.

The mere fact that a court thinks the reasoning is weak, unpersuasive, legally incorrect or incomplete does not ordinarily justify annulment.

The situation becomes different where there is effectively no reasoning, or where the purported reasoning is so disconnected from the decision that the award cannot meaningfully be understood.

STC 17/2021 recognizes that the annulment action may address circumstances including an award lacking motivation, but judicial review remains limited and cannot become a merits review.

Therefore:

Potentially problematic

“The respondent is liable. The claim is granted.”

with no explanation whatsoever.

Ordinarily sufficient

“The tribunal finds that the respondent breached Clause 12 because the contractual delivery date expired on X date, the agreed extension mechanism was not activated, and the documentary evidence establishes delivery only on Y date. The contractual defence under Clause 15 therefore does not apply.”

The second example provides an intelligible decisional chain even if one party believes the conclusion is legally wrong.

15. Internal contradictions in the award

Another important issue is contradictory reasoning.

An award can contain extensive reasoning but still create serious problems if:

• the factual findings contradict each other;
• the reasoning contradicts the dispositive section;
• the tribunal states that a contractual clause applies and subsequently reaches a conclusion incompatible with that clause without explanation; or
• the tribunal's reasoning makes the operative result impossible to understand.

Thus, length is not the same thing as adequate reasoning.

A 150-page award can be defective if its reasoning is internally incoherent.

Conversely, a 30-page award can be perfectly valid if it clearly identifies and resolves the decisive issues.

This is consistent with the Constitutional Court's emphasis on cognizability, rationality and absence of arbitrariness, rather than a fixed requirement of length.

16. Failure to deal with a decisive issue

Although a tribunal need not address every argument, it should ordinarily address material issues necessary to the disposition of the dispute.

For example, if the entire claim depends on whether a limitation clause is applicable, an award that decides liability but simply ignores the limitation clause may create a more serious reasoning problem.

The distinction is therefore:

Every argument ≠ every decisive issue.

The tribunal does not need to answer every subsidiary submission, but its reasoning should demonstrate that it has resolved the issues necessary to reach its operative conclusions.

17. Relationship with Article 41 of the Spanish Arbitration Act

Article 41 establishes the grounds on which an arbitral award may be annulled.

The reasoning requirement under Article 37(4) must therefore be understood together with the limited grounds for annulment.

Of particular importance is Article 41(1)(f), concerning public policy (orden público).

The Constitutional Court has repeatedly warned that public policy cannot become a mechanism for reviewing the substantive correctness of an award.

This produces a critical principle:

A judicial disagreement with the tribunal's reasoning is not, by itself, a public-policy violation.

This is one of the most important safeguards against converting annulment proceedings into appeals.

18. Judicial review: existence rather than correctness

The Spanish Constitutional Court's modern doctrine can be summarized as follows:

Tribunal's task

Determine the dispute and provide reasons.

Annulment court's task

Determine whether a statutory ground for annulment exists.

Annulment court's prohibited task

Re-decide the dispute.

This is why the Constitutional Court has repeatedly described judicial review as external and limited.

STC 65/2021 specifically states that the court should not examine the appropriateness, sufficiency or substantive correctness of arbitral reasoning merely because it disagrees with the arbitrator.

19. Six principal case laws — consolidated table

The 2026 decision is particularly significant because it expressly reiterates the principles developed in STC 17/2021, STC 65/2021 and STC 146/2024.

20. Practical drafting standard for a Spanish-seated award

Although Spanish law does not require exhaustive reasoning, a tribunal seeking to produce a robust award should normally follow this structure:

I. Identification of the dispute

Briefly identify:

• parties;
• arbitration agreement;
• applicable arbitration rules;
• seat;
• claims and counterclaims.

II. Procedural history

Record material procedural events, including:

• constitution of the tribunal;
• submissions;
• hearings;
• evidentiary proceedings;
• procedural orders;
• jurisdictional objections.

III. Issues for determination

State clearly what the tribunal has to decide.

This is particularly useful for demonstrating that the tribunal has remained within the scope of the arbitration agreement.

IV. Findings of fact

Identify the material facts and explain the evidentiary basis for important factual conclusions.

There is no requirement to discuss every document individually.

V. Applicable law

Identify the governing legal regime and the principal provisions relevant to the dispute.

VI. Analysis

Apply the law or equitable principles to the material facts.

VII. Conclusions

State clearly how the analysis resolves each material claim and defence.

VIII. Relief

Set out precisely:

• principal amounts;
• interest;
• declarations;
• specific performance, where applicable;
• costs.

This structure greatly reduces the possibility of a genuine reasoning defect while avoiding unnecessary verbosity.

21. What arbitrators should avoid

A Spanish-seated tribunal should particularly avoid:

1. Bare conclusions

“Claimant succeeds because the contract was breached.”

That is generally too conclusory.

2. Contradictory findings

The factual and legal sections should support the dispositive part.

3. Ignoring decisive issues

The tribunal should explain the treatment of issues that are indispensable to the result.

4. Inventing reasons after the fact

The award itself should disclose the reasoning. The tribunal should not depend on explanations later supplied through procedural correspondence.

5. Excessive reliance on formulaic language

Simply stating “having considered all evidence” without meaningful analysis is weaker than identifying the decisive evidentiary findings.

6. Excessive reasoning

Ironically, very lengthy reasoning can create additional risks if it contains contradictions or unresolved alternative analyses.

The objective should be sufficient, coherent and intelligible reasoning, rather than maximum length.

22. The core doctrinal distinction

The Spanish position can ultimately be reduced to four propositions:

First: Article 37(4) SAA requires arbitral awards to be reasoned.

Second: the requirement is principally a statutory requirement, not a constitutional requirement identical to the reasoning obligation applicable to judicial decisions.

Third: the tribunal does not have to answer every argument, identify every piece of evidence, or provide exhaustive reasoning.

Fourth: Spanish courts exercising annulment jurisdiction generally cannot replace the tribunal's reasoning with their own assessment of the evidence, law or merits.

Conclusion

Spain therefore adopts a moderate but meaningful standard of arbitral reasoning. The award must provide genuine reasons, and those reasons must make the tribunal's decisional path sufficiently understandable and non-arbitrary. But Spanish law does n