Previous studies on web-based collaborative writing have shown that task modality impacts peer interaction patterns and attention to form. However, these studies have primarily focused on contrasting a face-to-face oral condition with a text-based synchronous computer-mediated communication (SCMC) environment. Few studies have compared peer interaction patterns and attention to form between text-based SCMC and text-based asynchronous computer-mediated communication (ACMC), the two common modes of computer-mediated communication. This study aimed at filling this research gap. Participants were 22 EFL learners who worked in self-selected pairs to complete two L2 collaborative writing tasks, one through text-based SCMC and the other text-based ACMC, using Google Docs. Patterns of interaction for each pair within each task mode were identified. Attention to form was measured through language-related episodes (LREs). Quality of attention to form was assessed via learners' LRE engagement levels. The results show that task modality affected the occurrence but it did not affect the focus and outcome of LREs. Task modality, however, impacted peer interaction patterns and quality of attention to form, with more collaborative pair dynamics and elaborate LRE engagement being generated in text-based SCMC tasks.