Abstract In Task-Based Language Teaching (TBLT), there is clearly a need for a balanced and flexible view of L1 use. Considering this, a matter of concern in TBLT is that learners might overuse their first language (L1) while performing complex tasks. To test this speculation, the present study investigated the effects of manipulating task complexity along resource-directing variables (+/− reasoning demands) of the Cognition Hypothesis on L1 use and its functions. Further it aimed to test the impacts of task type on L1 use and its functions. To do so, the interaction of 24 intermediate EFL learners while completing two different task types with two levels of task complexity was analyzed. The analysis of L1 words and turns indicated that both task complexity and task type affected L1 use significantly. The results further indicated that L1 was employed for a variety of purposes which were mainly affected by task complexity.