This paper reports a case study investigating how the use of instant messaging (IM) mediated ESL students’ motives in their participation in computer-mediated peer response (CMPR) tasks in an ESL academic writing class. Qualitative data including interview transcripts, chat transcripts, on-and off-screen behaviors captured on video cameras and with a screen-capturing tool, researcher observations, and student drafts collected from two low-advanced-level ESL students were analyzed. Data analysis indicated that with the opportunities afforded and challenges presented by IM technology, the ESL students were driven by heterogeneous and multiple motives even when they were participating in the same task, and their engagement in multiple motives was dynamic rather than static. The use of IM mediated the formation and shift of students’ motives within and across the CMPR tasks. © 2010 Elsevier Inc. All rights reserved.