Revision constitutes an important component of the writing process that integrates text interpretation, reflection, and production. Although previous studies have offered useful insights into the revision behaviour of L2 writers at different levels of proficiency using off-line measures, little is known about the online processes of revision. In this study, I used keystroke logging to investigate longitudinal changes in the revision processes of postgraduate and undergraduate students in the context of an intensive English for Academic Purposes (EAP) program. Twenty-five postgraduate and 14 undergraduate students participated in this study. The keystroke logging program Inputlog was used during two essay writing sessions at the beginning and at the end of the course to observe how the participants revise their essays and to get insights into their on-line writing processes. Keystroke logging data were coded in terms of the orientation and location of revisions. The results revealed that postgraduates produced more content-oriented revisions at the end of the programme than at the beginning. The opposite trend was observed for the undergraduates, who revised their essays significantly less frequently at the end than at the beginning of the EAP program and made more mechanical revisions than postgraduate writers. The findings highlight fundamental differences in the developmental trajectories of revision processes between postgraduate and undergraduate students. These differences caused by previous academic writing experience and language proficiency should be considered in developing materials for EAP programs. In the paper, I also discuss benefits of using keystroke logging to investigate L2 writers' revision processes.