Previous research has noted an increased attention to language form as one of the many benefits of collaborative writing (CW). Although prior studies have investigated different variables (e.g. proficiency, pre-task modelling) to promote attention to form (often operationalized as a language-related episode, LRE), one crucial yet underexplored variable is whether a shared native language affects LRE use during CW. Researching this question informs an understanding of how L1 use influences the potential of CW in L2 classrooms. The present study considers LREs in both L1 and L2 use conditions. Thirty-five pairs of EFL learners were asked to complete two argumentative writing tasks collaboratively. In the first task, one group interacted in the L1 and another group interacted in the L2. In the second task, the groups switched the language used for interaction and wrote on a similar topic. Results showed that interacting in a shared L1 affords learners more opportunities to discuss and resolve lexis and mechanics related issues. Furthermore, learners in the L1 use condition were able to negotiate language use in a more elaborate manner and use a wider range of L2 lexical items. In light of these findings, the pedagogical implications of L1 use during CW are discussed.