Obesity and sedentary lifestyles are worldwide threats to public health but feasible and effective strategies to tackle these problems are scarce. The identification of variables that explain why only a sub-group of overweight persons succeeds at increasing physical activity and achieving long-term weight control is a key research topic in exercise and sports sciences. Subjects are 260 healthy women (BMI, 25 to 40 kg/m2), aged between 25 and 50 years, premenopausal, of whom half receive the treatment intervention, the remaining being randomly assigned to a control group at baseline. This RCT consists of a 1-year intervention plus a 2-year no contact follow-up period. The intervention group attended 30 weekly group sessions, designed to follow SDT basic tenets, covering PA, eating/nutrition, body image, and other cognitive and behavioral contents. Central to the intervention model are autonomy, intrinsic motivation, and self-regulation of behavior change. Autonomy-supportive treatment climate and internal causality orientations are predicted to enhance autonomous regulation, perceived competence, and intrinsic motivation for the targeted behaviors, resulting in lasting behavioral and body weight changes. The intervention was designed to follow SDT, with an emphasis on promoting intrinsic, self-regulated motivation for exercise and weight control. The control group received a general health education curriculum based on several 3- to 6-week long educational topics (e.g. food safety, stress management, self-care, body image, and others). Results are expected to contribute to a better understanding of how individual characteristics, particularly those related to physical activity and exercise influence success, better screening/readiness testing procedures, improved matching of interventions to participants, and lower rates of attrition and unsuccessful weight loss attempts.