This paper examines the line profile evolution due to bulk motion in circumstellar envelopes during microlensing fold caustic crossing events. These events have recently been shown to be a sensitive probe of stellar surface brightness profiles, thus providing a means - through both photometric and spectroscopic observations - to constrain and test stellar atmosphere models. Here it is demonstrated, through the examination of simplified line profiles, that spectroscopic studies of fold caustic crossings could also prove to be a powerful diagnostic of bulk motion in circumstellar envelopes.