This research examines the evolutionary patterns and determinants of multiplex organizational communication networks. Based on the data between 1997 and 2005 collected from the records of development projects in the field of Information and Communication Technology for Development, the study demonstrates that dynamics in one network are significant drivers of tie formation in the other network at both dyadic and triadic levels. In particular, results show that the effects of common third-party ties and structural embeddedness exist across multiplex networks. Further, the study suggests that resource similarity of organizational dyads, resource width, and organizational centrality have positive effects on the propensity for multiplex ties. These results have implications for organizations' communication networking strategies in a wide variety of organizational communities.