Background Group exercise courses are popular among adults, but dropout rates are high. Studies of relationships between participants’ perceptions and their participation might highlight factors to target to improve adherence and re-enrolment.

Methods We used a mixed-methods approach to analyse perceptions of group exercise courses and instructors among 463 adults. Participants completed the Exercise Barriers and Benefits Scale, questionnaires on perceptions of the instructor and course, and non-participation. We assessed participation from weeks 2–4 and 5–10, and re-enrolment. We analysed relationships between perceptions and re-enrolment using linear regression and mediation analyses. We conducted group interviews with 11 participants.

Results Predictors of re-enrolment included early participation (β=0.11, P=0.029) and perceptions of the group social climate (P=0.027). Perceptions of the group mediated the relationship between early participation and re-enrolment (95% CI 0.0036 to 0.0471): early participation predicted more positive perceptions (β=2.11, P=0.003), which predicted re-enrolment (β=0.01, P=0.006). Qualitative analyses highlighted instructors’ roles in promoting social exchange and integrating participants into the group.

Conclusions The social climate of group exercise courses is a key factor predicting re-enrolment. Early participation predicts re-enrolment on its own, and also promotes positive perceptions of the group. Instructors can target these factors by sensitising participants to the importance of early participation, and promoting social exchange.