This research investigated the role of a commercial program, Michelle Bridges 12 Week Body Transformation (MB12WBT), that uses a variety of tools including social support to influence weight management behaviours such as exercising and eating healthfully. The need for research into weight management remains current given obesity rates have continued to increase in recent decades. Weight management is a complex behaviour that for many consumers involves barriers that influence their commitment to continue exercising and eating healthily. The Theory of Planned Behaviour is frequently used to explain, and to a lesser extent predict, behaviours such as physical activity and healthy eating; however, there is much debate as to whether a perceived behavioural control variable or self-efficacy is the best predictor of behaviour. Structural equation modelling was used to analyse data from a sample of 724 respondents who answered a baseline and three month follow-up survey. The analysis revealed self-efficacy and perceived behavioural control were two distinct constructs. Self-efficacy was a better predictor of behaviour than perceived behavioural control variables as suggested in the original Theory of Planned Behaviour. Furthermore, self-efficacy had a significant impact on exercising and eating healthfully behaviour. The findings and implications for both behaviour change theory and practice are discussed.