This paper examines the relationship between the work of James Buchanan and Vincent Ostrom. It adds to existing work by providing a comprehensive analysis of Buchanan’s and Ostrom’s changing views about how the ‘logical foundations of constitutional democracy’ should be conceptualised. The paper traces how in the 1960s and 1970s Ostrom took inspiration from the rational choice analysis of constitutional democracy in Buchanan and Tullock’s The Calculus of Consent, explaining how it shaped his reading of key texts in political theory and his analysis of public administration. It then discusses how Buchanan subsequently drew on Ostrom’s notion of artifactual man in developing his understanding of the ‘constitutional attitude’ necessary for individuals to engage in institutional design. It then explores how, from the mid-to-late 1990s, Ostrom became increasingly critical of Buchanan’s reliance on rational choice theory for his analysis of constitutional decision-making, identifying this as a key difference between their views.