This paper reports on a quasi-experimental study comparing the effects of peer-editing to self-editing on improving students' revised drafts. The study involved two intact classes (experimental and control groups) of an English course. The experimental group practiced peer-editing while the control group engaged in self-editing. After receiving sufficient training in their respective type of editing, both groups wrote a graded argumentative essay in two drafts. Results of a MANCOVA test carried out on the graded essay written by the two groups showed a statistically significant difference in revised writing in favour of peer-editing. A random sample of seven peer-edited and self-edited essays was analyzed to determine the differences between peer-editors' and self-editors' ability to notice errors, revise, and improve them. Results revealed that while peer-editors and self-editors had more or less the same noticing ability, writers who engaged in self-editing revised more errors than writers who received peer-feedback. In contrast, writers who engaged in peer-editing improved their revised drafts more than self-editors did. Differences in revised writing performance between the two groups are attributed to the use of language learning strategies, peer interaction, and engagement with language. The paper concludes with implications for classroom teaching/learning and recommendations for future research. (Contains 14 tables.)