The relationship between contractual and relational arrangements in interorganizational relationships has been subject to an ongoing debate. We propose that in the context of cross-border partnerships, the governance mechanisms can be both substitutes and complements depending upon contingencies posed by uncertainties of two different origins: environmental and behavioral. We argue that environmental uncertainty (i.e., instability and unpredictability of the external environment) drives the formal and relational arrangements into a more substitutive relationship by elevating the adaptation complications in which increasing reliance on either form of governance inhibits the effective operation of the other. Contrastingly, behavioral uncertainty (in the form of inadequate common grounds and shared frameworks among collaborating firms) encumbers the understanding of partner behavior and conduct and drives the governance mechanisms into a more complementary relationship in which contractual and relational mechanisms facilitate the effective operation of each other. Empirical results from 205 cross-border partnerships of large U.S. firms support our theorized relationships.