This study examines whether task-modality (speaking vs. speaking+writing) influences first language (L1) use in task-based English as a foreign language (EFL) learner-learner interaction. Research on the topic has shown that different task-modality triggers different learning opportunities with collaborative speaking tasks drawing learners' attention to meaning and tasks that also incorporate a written component drawing attention more to formal linguistic aspects. Research has also shown that a balanced L1 use might be positive in learner-learner interaction, as it helps learners maintain their interest in the task and acts as a strategy to make difficult tasks more manageable. This article analyses L1 use and the functions it served during the oral interaction of 44 EFL Spanish learners while they completed four collaborative tasks: two speaking tasks (picture placement and picture differences) and two speaking+writing tasks (dictogloss and text editing). Findings point to a clear impact of task-modality on L1 use, as speaking+writing tasks made learners fall back on their L1 more frequently. L1 functions were also task dependent with grammar deliberations more frequent in speaking+writing tasks and vocabulary searches in speaking tasks.