Although research about group interactions during second or foreign language (L2 or EFL) collaborative writing has proliferated in the last few decades, little is known about the role of psychological factors, like learners' knowledge about collaborative writing, in affecting students' patterns of interaction and learning in collaborative writing. Informed by metacognitive theory, this study used a mixed-method approach to investigate whether and how L2 students' knowledge about collaborative writing affected their participation (patterns of interaction) and learning (languaging opportunities) during the collaborative writing process. Two parallel classes (one with explicit collaborative writing knowledge taught; one without) were compared and multiple sources of data (interviews, reflective journals, pair talk) were analysed for patterns of interaction, quality and quantity of language-related episodes (LREs). The statistical analyses of the data indicated that the participants who were provided more exposure to knowledge about collaborative writing tended to exhibit more collaborative patterns of interaction and produce more LREs. Qualitative analyses revealed that knowledge about collaborative writing affected participation and learning during three distinct stages: planning, writing, and revising. This study fills a gap in collaborative writing research to include a metacognitive theory perspective and sheds new light on L2 collaborative writing pedagogy.