The legality, legitimacy, and constitutional dimension of transfer pricing law concerns the threshold question of whether domestic legislation empowering tax authorities to adjust intercompany pricing is itself valid, proportionate, and enforceable. Unlike disputes over methodology or comparability, these cases challenge the legal foundation of the adjustment power itself — questioning whether the enabling statute, as drafted or applied, respects constitutional protections such as legal certainty, the principle of legality in taxation, non-retroactivity, and the right to property. The disputes arise across jurisdictions with distinct constitutional traditions but share a common structure: a taxpayer or legal body contends that the tax authority’s adjustment power lacks a sufficient legal basis or is otherwise unconstitutional.

In practice, these challenges arise when taxpayers exhaust administrative and appellate remedies and escalate to constitutional or supreme courts. In the Portuguese cases involving A Const S.A. and ALP S.A., taxpayers challenged the constitutional validity of Article 57 of the CIRC, the domestic arm’s length provision, after losing on the merits. In Peru, a bar association sought to invalidate Legislative Decree 1421’s statute of limitations provisions on constitutional grounds, arguing that collection rights against large taxpayers were improperly extinguished. The Kenyan High Court in Unilever Kenya was asked to assess whether the Commissioner’s adjustment powers under the Income Tax Act were exercised within lawful bounds. Tax authorities generally prevail where the enabling legislation is clear and proportionate, while taxpayers succeed where procedural safeguards are absent or powers are applied beyond their statutory scope.

The OECD Transfer Pricing Guidelines do not directly address constitutional law, which remains a matter of domestic legal order. However, the arm’s length standard in Article 9 of the OECD Model Tax Convention establishes the international normative foundation that most domestic provisions seek to implement. Where a domestic provision departs materially from that standard, or is applied without procedural regularity, constitutional arguments gain traction. Commentary to Article 9 and the OECD’s guidance on secondary adjustments and burden of proof (Chapter III of the Guidelines) are relevant background when courts assess proportionality.

Courts in these cases focus on whether the adjustment power is grounded in sufficiently precise statutory language, whether the taxpayer received adequate notice, and whether the authority’s discretion was exercised within defined limits. Evidence of administrative overreach, retrospective application, or departure from published methodologies often supports the taxpayer. The McDonald’s Spain case and the Greek ST. Medical decision illustrate how procedural and substantive irregularities intersect with challenges to the legitimacy of the adjustment itself.

This category matters because a finding of constitutional invalidity renders an otherwise correct transfer pricing adjustment unenforceable, making it the highest-stakes dimension of the field.