Organizations need to collaborate to achieve complex goals. Although interorganizational relations often take the form of multiplex ties, our understanding of how multiplexity itself may facilitate interorganizational collaboration is limited. We use dynamic network analysis (SIENA) to test the role of relational multiplexity – specifically, relationships involving communication outside of coalition meetings and expertise-seeking – in promoting collaboration in a health justice coalition over three years. The results offer strong support for the role of multiplexity in the formation of interorganizational collaboration, indicating that having multiple ties between organizations facilitates collaboration, and that certain types of ties (i.e. communication relationships outside of coalition meetings) are more influential than others. We conclude that coalitions hoping to support successful interorganizational collaboration will benefit from offering opportunities for member organizations to communicate outside of group meetings, because such small acts of dyadic interaction can build into deeper levels of engagement. Additionally, our study demonstrates how network analysis can help organizational coalitions to track and suggest potential partnerships between member organizations.