Alliance research emphasizes that firms can access research and development (R&D) collaboration opportunities when they enjoy relational or geographic embeddedness with potential partners. However, how can firms that are not embedded with prospective partners establish alliances? We emphasize the microfoundations of R&D alliance formation and propose that scientist mobility is an important substitutive mechanism that helps foster collaboration opportunities between firms that are poorly embedded. Specifically, we posit and show that in high-tech industries, scientist mobility is more facilitative for R&D alliance formation when potential partners lack relational ties between them or are not geographically colocated. Our findings demonstrate how incorporation of the competitive labor market context and its interplay with the cooperative context significantly changes the insights of a fundamental research stream emphasizing the importance of the cooperative context for alliance formation.