This study investigated the effect of repeated in-class web-based collaborative writing tasks on second language writers' (L2) individual writing scores. A pre-test post-test research model was used in addition to participant surveys, class observations, and teacher interviews. Participants included 59 L2 writers in a writing class at a large U.S. university. The 32 participants in the experimental group engaged in four in-class web-based collaborative writing tasks, while the 27 participants in the control group engaged in the same four in-class web-based writing tasks but individually. A paired samples t-test revealed that both groups experienced statistically significant gains from their pre- to posttest scores. An independent sample t-test of pre- to post-test gains revealed that the participants in the collaborative web-based writing group experienced statistically significant writing gains in their individual writing over the participants in the individual web-based writing group. Participant survey results showed that the L2 writers valued the collaborative in-class writing tasks overall and that many participants in the individual group wished they had done in-class collaborative web-based writing. Three types of collaborative groups emerged. Pedagogical implications for technology-enhanced collaborative writing are discussed, and a Teaching Cycle for Web-Based Collaborative Writing is introduced.