This study empirically examines the general propositions of media capacity theories using a newer and increasing popular communication medium: Instant Messaging. We developed hypotheses based on the proposition that synchronous communication media would be perceived to be more effective for convergence communication and asynchronous communication media would be 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 suggested that although IM was perceived to be a highly synchronous communication medium, it was not perceived to be as effective for convergence communication as for conveyance communication. These results challenge the general propositions of media capacity theories. Implications of the findings are discussed. .