The present case study examined factors that mediated interaction among 3 Asian learners of English while they collaboratively wrote a summary using Google Docs and text-chat (Task 1) and Google Docs and voice-chat (Task 2) in the context of a debate club. Data were collected from multiple sources including a survey questionnaire, debate summaries, screen recordings, and stimulated recalls. The survey questionnaire elicited the learners’ background information and individual goals for the writing tasks. Debate summaries, transcripts of screen recordings, and stimulated recall interviews were analyzed to investigate interaction patterns of the group and mediating factors in their collaborations. Firstly, the group’s interaction patterns were identified by adapting Storch’s (2002) dyadic interaction model, revealing a facilitator/participants pattern in Task 1 and a collaborative pattern in Task 2. Informed by activity theory, participants' goals and the goal-directed actions that influenced their collaborative writing activities were identified. Drawing upon an expanded activity model, the findings suggested that modes of communication, task representations, matches/mismatches between participants’ self-perceived and other-perceived roles, and perceptions of peer feedback were the primary mediating factors on the qualities of collaboration. The findings may help explain why collaborative performance varies and may provide insights into how web-based collaborative writing activities can be designed and facilitated in L2 classes.