This study investigated second language (L2) learners' use of first language (L1) as a means to resolve lexical issues during collaborative writing tasks (i.e., L1 languaging). Participants were 34 8th grade high-proficiency English learners in Chile who shared their L1 (Spanish). Their vocabulary development was tested via the vocabulary knowledge scale (VKS). Their interactions during the collaborative writing tasks were audio-recorded. The transcripts were first analysed in light of L1 focus such as content, language, and task management. Then, lexical language-related episodes (LREs) involving L1 were examined. The regression models of the VKS and LRE scores showed that L1 languaging was associated with higher gains in vocabulary knowledge.