Purpose: Dietary adherence is a significant issue in treating obesity and prevention of heart disease using energy-restricted diets. Such diets are efficacious in short-term trials in controlled environments, but are not effective in longer term community based settings due to drop out and poor dietary adherence. Identification of patients most likely to adhere to treatment diets represents a cost-effective way to use this treatment modality. This study applied self determination theory to investigate adherence in an Australian setting in a monounsaturated enriched weight loss diet trial. Methods: Non adhering and adhering participants were interviewed about their dietary adherence motivations at one month and these were analysed using thematic analysis. Results were integrated with quantitative demographic data and self determination theory health scales within a concurrent transformative mixed methods research design. Results: Key determinants of adherence were identified as a positive dietitian/nutritionist who was supportive, friendly and non-critical; supportive interactions including praise, or practical help, and; maintaining a positive mind-frame through re-focusing. 'Falling off the wagon' and 'failure' were identified as barriers by participants in both groups. Non-adherers had greater focus on these and associated negative feelings. Non-adhering participants also felt 'out of control' of their ability to lose weight. Emotional management problems such as 'bad days' were another key theme although adherers identified finding strategies to cope with these. Conclusions: Findings suggest strong, persistent themes affecting dietary adherence, which could be incorporated into protocols of recruitment and intervention to potentially enhance adherence to, and cost-effectiveness of, community-based weight loss trials.