ObjectivesTo understand the barriers and facilitators to uptake and retention of postnatal women randomised to a commercial group weight management intervention using the COM-B (capability, opportunity, motivation and behaviour) behaviour change model.DesignConcurrent mixed-methods (qualitative dominant) process evaluation nested within a feasibility randomised controlled trial, comprising questionnaires and interviews at 6 and 12 months postbirth.SettingOne National Health Service maternity unit in an inner city area in the south of England.Participants98 postnatal women with body mass indices>25 kg/m2 (overweight/obese) at pregnancy commencement.InterventionTwelve-week Slimming World (SW) commercial group weight management programme, commencing anytime from 8 to 16 weeks postnatally.Primary and secondary outcome measuresData regarding uptake and retention from questionnaires and interviews conducted 6 and 12 months postbirth analysed thematically and mapped to the COM-B model.ResultsBarriers to SW uptake mostly concerned opportunity issues (eg, lack of time or childcare support) though some women also lacked motivation, not feeling that weight reduction was a priority, and a few cited capability issues such as lacking confidence. Weight loss aspirations were also a key factor explaining retention, as were social opportunity issues, particularly in relation to factors such as the extent of group identity and relationship with the group consultant; and physical opportunity such as perceived support from and fit with family lifestyle. In addition, barriers relating to beliefs and expectations about the SW programme were identified, including concerns regarding compatibility with breastfeeding and importance of exercise. Women’s understanding of the SW approach, and capability to implement into their lifestyles, appeared related to level of attendance (dose–response effect).ConclusionsUptake and retention in commercial weight management programmes may be enhanced by applying behaviour change techniques to address the barriers impacting on women’s perceived capability, motivation and opportunity to participate.Trial registration numberISRCTN39186148.