Considerable evidence has indicated that group-based physical activity may be a promising approach to reducing and preventing age-related illness. However, this research has not examined the mechanisms by which cohesion may impact on behaviour. The purpose of the present research was to utilize the theory of planned behaviour to investigate the mechanism by which group cohesion may affect physical activity intention. Participants were recruited from an existing physical activity intervention studying the effects of group cohesion on physical activity behaviour. The outcomes of this intervention are reported elsewhere. This paper presents data from a sub-sample of the intervention population (N=74) that examined the mediating relationships between the theory of planned behaviour and group cohesion on physical activity intention. Analyses showed that attitude and perceived behavioural control mediated the relationship between specific group cohesion concepts and physical activity intention. The direct measure of subjective norm failed to display a mediating relationship. The mediating relationships displayed between attitude and perceived behavioural control and physical activity intention provide insight into potential mechanisms by which group cohesion may affect behaviour.