Two studies analyzed impacts of writing and receiving web-mediated peer reviews on revision of research reports by undergraduate science students. After conducting toxicology experiments, 77 students posted draft reports and exchanged double-blind reviews. The first study randomly assigned students to four groups representing full, partial, or no peer review. Students engaging in any aspect of peer review made more revisions than students confined to reviewing their own reports. In the second study, all students engaged in peer review, and the influence of writing versus receiving critiques was analyzed using linear regression. Both studies showed receiving reviews to be more significant than writing them in terms of triggering report revisions. Students valued the peer review experience and credited it with giving them insights about their work. Conclusions address implications for optimal design of online peer review systems and for further research into student learning gains.