This study empirically examines the general propositions of media capacity theories using a newer and increasingly popular communication medium: instant messaging (IM). We develop hypotheses based on the proposition that synchronous communication media are perceived to be more effective for convergence communication while asynchronous communication media are perceived to be more suitable for conveyance communication. These hypotheses were tested using data collected from 81 IM users in four organizations by means of survey instruments developed for this study. The results suggest that although IM was perceived to be a highly synchronous communication medium, it was not perceived to be as effective for convergence communication as it was for conveyance communication. These results seem to challenge the general propositions of media capacity theories. Theoretical implications are discussed.