It is important to develop understanding of what underpins the engagement of students in online learning environments. This article reports on a multiple case study that explored student engagement in a set of postgraduate degrees offered on a fully online basis. The study was based on a theorization of student engagement as the exercise of intentional human action, or agency. It identified ways in which tasks and social relations in the online learning environments triggered reflexivity on the part of students, with "reflexivity" understood to mean the ordinary mental capacity to consider oneself in relation to one's social setting. A different relationship between reflexivity and student engagement was in view than that identified by Margaret Archer with regard to reflexivity and social mobility. Rather than displaying one dominant mode of reflexivity, the students considered in the study were seen to draw on a range of modes. The engagement of these students in their learning was also seen to depend on the manner in which they engaged in reflexivity centred on the pursuit of shared goals, that is in collective reflexivity. Specific practices were seen to trigger constructive forms of collective reflexivity, while fractured and restricted forms of collective reflexivity were linked to student disengagement in relation to joint tasks. As well as adverting to the importance of collective reflexivity to learning, the study highlights scope for dissonance between the modes of reflexivity and practices favoured by an online learning environment and the reflexive profile of the student.