Equitable Principles as Deus ex Machina: Illegality and Contract Validity in Investment Contract Arbitration

Abstract Investment contracts serve as critical instruments for structuring collaborations between states and foreign investors, often incorporating concessions that limit a state’s sovereign regulatory authority. Given their potential impact on public interests, these contracts are frequently subject to domestic legal constraints. How arbitral tribunals address these constraints, and whether they treat them as affecting contractual validity, is far from consistent. This article examines arbitral practice, showing how tribunals address mandatory law claims in contract-based investment arbitration. It offers a critical analysis of the application of estoppel and good faith principles in denying a state’s right to invalidate investment ­contracts. Tribunals often deploy these principles as autonomous norms rather than doctrines whose operation depends on the applicable law. This shift from legality to equity enables tribunals to override domestic constraints without articulating a supporting legal basis. The article contends that such equity-driven reasoning undermines party autonomy and risks eroding the legitimacy of investment arbitration.