This article critically discusses the recourse to the principle of restrictive interpretation (in dubio mitius) by treaty-based investor-state arbitral tribunals. Although its status as a rule of international law is at best controversial, in dubio mitius has been applied by a number of arbitral tribunals interpreting umbrella clauses and most-favoured-nation (MFN) provisions contained in investment treaties. This article shows that restrictive interpretation is inappropriate and undesirable. It highlights, first of all, that no rational justification for in dubio mitius exists and that the sovereignty-based rationale put forward is obsolete, illogical and largely dysfunctional. It also shows that restrictive interpretation frequently clashes with other, more fundamental, rules of treaty interpretation and that in dubio mitius interpretation of investment treaties has an inherently discriminatory effect on investors.