Student engagement in the second language classroom has been the focus of numerous researchers and teachers. Previous studies have shown that there are several dimensions of student engagement, but it is still unclear how they change (or not) over time and consequently how they affect actual task performance. This study investigated the task engagement of language learners engaged in collaborative writing in pairs. Specifically, it focused on the combination of behavioral, cognitive, emotional, and social dimensions of task engagement, and examined which combinations resulted in better task performance. Participants were 60 Japanese university students who worked in pairs on a picture description task. Multiple data sources, such as the number of words/turns/language-related episodes, patterns of dyadic interaction, and self-reported questionnaire results, were utilized to investigate the process of students' task engagement. The results showed: that the 30 participating pairs fell into three groups showing similar combinations of dimensions; that there was a significant difference in actual engagement between the groups and across time; and that such differences had a significant impact on task performance. Based on the results, pedagogical implications for teachers are discussed concerning the use of pair work in the language classroom.