MST

The Minimum Standard of Treatment (MST) is a doctrine of customary international law requiring states to accord foreign nationals — and, in modern investment law, foreign investors and their property — a baseline level of treatment regardless of how the state treats its own citizens. It is distinct from national treatment (equal treatment with locals) and most-favoured-nation treatment (equal treatment with other foreigners), and can be invoked even when those relative standards are met.

The classical formulation comes from the Neer v. Mexico case (US–Mexico General Claims Commission, 1926), which held that state conduct toward an alien must not amount to "an outrage, to bad faith, to wilful neglect of duty, or to an insufficiency of governmental action so far short of international standards that every reasonable and impartial man would readily recognize its insufficiency." Whether that high threshold still applies, or whether MST has evolved with contemporary practice, remains contested.

In modern investment treaties, MST typically appears as a Fair and Equitable Treatment (FET) clause, often coupled with full protection and security. NAFTA Article 1105 is the best-known example, and the NAFTA Free Trade Commission issued a binding 2001 interpretation clarifying that Article 1105 reflects the customary international law minimum standard and does not require treatment additional to or beyond it. The successor USMCA (2020) retained a similar formulation in Article 14.6.

Tribunals have read MST/FET to prohibit:

• denial of justice in domestic courts
• manifestly arbitrary or discriminatory measures
• breaches of an investor's legitimate expectations grounded in specific state assurances
• lack of due process and transparency

Notable arbitral interpretations include Waste Management II v. Mexico (2004), Glamis Gold v. United States (2009), which reaffirmed a high Neer-style threshold, and Bilcon v. Canada (2015). Capital-exporting states generally favour an evolutive reading; capital-importing states and some scholars argue this expands obligations beyond what custom supports.

Note: "MST" can also denote Brazil's Movimento dos Trabalhadores Rurais Sem Terra — a domestic social movement, not a treaty body.