This study examined the performance of university foreign language learners on a writing task by comparing paired work (n = 50) against individual work (n = 50). Data were collected based on three aspects of learners' task engagement: behavioural (time spent working on the task, number of words written), linguistic (English writing scores), and affective (attitudes toward the task). The results showed that (1) all participants worked on the task for the same amount of time and produced a similar number of words, (2) texts written by pairs were similar to and as accurate/fluent as those written individually, and (3) learners who worked in pairs evaluated the task more positively than did those who worked individually. A more detailed examination using cluster analysis revealed that while there was generally a positive relationship between each aspect of task engagement, a trade-off might occur depending on the situation, and one aspect might mediate the effect of others. In addition, by examining the differences between a successful and a less successful pair, this study provides pedagogical implications in relation to the use of pair work activities in the language classroom.