This study empirically examines the determinants of heterogeneous firm-level cooperative R&D commercialization strategies. While the volume of interfirm collaboration has increased dramatically in recent decades, the determinants of firm-level choices among alternate modes of such cooperative activity remain relatively understudied. We develop a conceptual model of factors determining collaborative mode choice at the organizational portfolio level. These factors include the firm-level appropriation environment, in which deal-level choices have portfolio-level spillover implications, as well as governance capabilities developed by the firm over time. Using a random sample of innovating biotechnology start-ups, we assemble a firm-year panel dataset that aggregates transaction-level collaboration data to the firm-year level, allowing us to characterize firms' portfolios of collaborative deals. We find broad empirical support for our model, suggesting that a firm's appropriation environment and governance capabilities strongly influence portfolio-level collaboration mode choices. In addition, we explore the implications of governance capability development, finding that experience with particular modes, as well as deviations from existing capabilities, impact firm valuation.