Despite previous research suggesting that first language (L1) use fulfills important functions in collaborative writing (CW) tasks, research has yet to examine whether L1 or second language (L2) use may lead to variation in the lexico-grammatical aspects of learners' collaborative texts and the pair talk. Using a corpus-based approach, this study examined how interacting in the L1 or the L2 during CW tasks influenced the use of lexico-grammatical features in learners' co-constructed texts and the focus areas of their pair talk. The results suggest that L1 interaction significantly facilitates the production of lexico-grammatical features typical of academic writing in learners' co-constructed texts. Additionally, compared with L2 interaction, L1 interaction allows learners to focus more on language and task management in pair talk. Methodological and pedagogical implications are discussed.