In recent years, the development of a constructivist theory of international economic cooperation has been impeded by the persistent influence of classical assumptions that cast monetary structures as essentially material and treat state and societal interests in economic cooperation as exogenous to interaction. In this paper, I counter that a truly constructivist approach to the study of the international political economy must be based on more socialised Keynesian assumptions, which recognise that socially constructed `conventions' give meaning to economic incentives and thereby shape state and societal interests. I further offer an explicitly Keynesian-constructivist theory of monetary interests in cooperation, arguing that such an approach would subsume approaches that obscure the potential emergence of state or societal interests in cooperation.