Interventions in community or naturalistic settings targeting exercise and health eating have been difficult to implement. Making such changes simultaneously in both of these areas is difficult for clients.This study evaluated the effectiveness of a 52 week, non-dieting, PA focused lifestyle intervention programme, using Blue and Black’s (2005) evaluation framework and tenets of the Self-Determination Theory (SDT) to identify intervention mechanisms that are more efficacious and conducive for adherence, retention, and prevention of relapse. Participants. 62 healthy, pre-menopausal, predominantly white (97%) obese (BMI ≥ 30.0 kg∙m-²) females (age 24-55 years) randomly assigned to an intervention or a delayed start control group. Intervention Phase - 12 weeks: Exercise: Required 4 hours/week: Healthy Eating Sessions: Food practices; shopping and cooking (1x week for 3 weeks); Brief Cognitive Behavioural Therapy (Group session): 3 weeks intensive intervention focusing on eating behaviours; Educational Sessions: (1x week for 12 weeks); Physiology & psychology of exercise & eating behaviours; Healthcare Professionals Provided an Intervention Rationale in Accordance with SDT: Maintenance Phase - 40 weeks: Exercise Only for 4 hours/week: Tai Chi, Aqua aerobics, Circuit Classes x 2; Social Support: Follow up phone calls if missed 2 weeks of exercise sessions. Attrition, attendance, and participants’ evaluations of treatment helpfulness were also monitored. Results. Participants completing the 52 weeks lifestyle intervention showed significantly improved psychological functioning without significant weight change.Theoretically derived outcome measure of “Autonomy” & “Impersonal” orientation correlated with dimensions of general well-being and self perceptions. Regression  showed that Autonomy and Chance scales predicted adherence to the programme.AlsoSelf-determination Index predicted adherence at 52 weeks.Conclusion: It is recommended that psychological and physical profiling of individuals to be carried out prior to actual lifestyle intervention to identify those who will likely to need additional, psychological intervention to enhance their responsiveness to lifestyle interventions.