While learners' engagement has been recognized as important for second language (L2) learning in task-based language teaching (TBLT), how engagement is manifest in learners' L2 use during task performance and how tasks can be designed to facilitate better engagement have not received enough attention in the L2 research. This study investigates the relationship between learners' expressed preferences for two communicative L2 tasks and their engagement in L2 use during task performance. Twenty-one learners of English as a second language (ESL) performed two distinct tasks and participated in an interview after finishing them. This article compares behavioral, cognitive, and social measures of learners' engagement in L2 use while performing the task they preferred more and the task they preferred less as well as their expressed reasons for these preferences. The findings revealed a higher level of cognitive engagement in L2 use (i.e. greater negotiation of meaning and form) during the performance of the task they preferred more. The findings also revealed that the topic as well as opportunities to create ideas and address a genuine communicative need were the most important factors in determining learners' expressed task preferences. Implications for task design include allowing learners to create content for the tasks they perform and choosing topics that are familiar, personally relevant, and emotionally engaging to learners.