The current study examines determinants of course engagement for adult learners registered in a Massive Open Online Course (MOOC). It is framed under investment theories of adult intellectual development, which posit that effort and attention toward knowledge acquisition and maintenance is directed by both ability (e.g., reasoning ability) and non-ability determinants (e.g., personality; Ackerman, 1996). Although instructors of MOOCs may provide course learning objectives and a learning schedule, learners have choice and control over their learning experience. Learners can choose the topics in which they engage, the course materials to focus on, and whether or not they complete the course. To examine course engagement as it occurs in a five-week long MOOC, learners’ survey responses were linked to objective course data. Outcomes examined were course activity participation, course performance, and course completion. A model is proposed and tested in which these outcomes are predicted by individual differences in trait complexes (i.e., constellation of personality and achievement goal orientations) and more proximally by interest, prior experiences, prior knowledge related to course content, and course affective engagement. We found that an intellectual/mastery trait complex was predictive of course affective engagement, which in turn was predictive of course outcomes. However, we found no such relationships for a traditional/avoidant trait complex.