This study investigated how one type of learner-generated information and content, i.e. inter-group peer online feedback, provided on a wiki writing assignment for an English for Academic Purposes (EAP) course, could contribute to student L2 writing. Using a mixed method approach, more than 1000 entries of online peer comments were collected, coded, analysed and compared with more than 400 comments provided by the course teacher, as shown in the marked final writing. Correlation and chi-square tests were employed to measure the relationship between the tabulated frequency of comments and essay scores. In addition, an online survey was carried out to collect student perspectives. The results show that (1) peers try to be friendly, supportive and avoid face threatening when providing online feedback in an open learning environment; (2) peer critical comments can predict well the final score received from the course teacher; (3) peer feedback focuses more on content/meaning rather than form (language) issues in EAP settings; (4) peer feedback contributes to EAP writing in various ways, both tangibly and affectively. It is suggested that more caution and thinking should be involved in designing the wiki learning environment judiciously by considering a number of factors that can prompt peers to provide appropriate and useful inter-group comments to help other groups improve their writing and L2 learning.