The standard of review to be applied by investment treaty tribunals when reviewing host state conduct is a crucial, but still insufficiently analysed issue. Although tribunals frequently make reference to ‘deference’ as the applicable standard, the criteria they apply to concretize that concept remain uncertain and little predictable. What is more, the conceptual foundations of granting deference to host states are opaque. The present article focuses on these foundations and argues that they are intrinsically connected to how investment treaty arbitration is qualified as either a form of commercial arbitration, a means to settle disputes under public international law, or as an internationalized form of judicial review in public law disputes. Siding with the latter, the present article proposes to conceptualize the standard of review within a separation of powers-framework that fuses domestic and international legal considerations in allocating power between states and arbitral tribunals. Within this framework, considerations originating from both international dispute settlement and comparative public law interact to determine and concretize the standard of review.