Materially powerful states tend to dominate both the creation of international organizations (IOs) as well as subsequent IO policymaking. Materially weak states are nevertheless expected to participate in IOs since it is generally assumed that they will still profit from cooperation and prefer power to be exercised through institutions. Yet, we know surprisingly little about how exactly institutional rules protect weak states from the powerful in IOs. This paper develops a theory of institutional design that specifies the institutional power equilibrium at the heart of IOs’ constitutional treaties. Through the inclusion of veto or exit rights, weak states obtain formal safeguards against exploitation by the powerful during an IO’s operation. This expectation of a power equilibrium in IOs’ design is borne out in design patterns within the constitutional treaties of IOs created between 1945 and 2005. Our results indicate that the distribution of power among an IO’s founding members indeed affects the inclusion of institutional safeguards in their constitutional treaties and that veto and exit rights are functional substitutes in this regard. Our findings matter since the institutional power equilibrium at IO creation has important implications for relations between the materially powerful and the weak during IO operations.