Objective: The research tested the efficacy of planning and partner-based interventions to promote physical activity over six months. Method: Local government (council) employees (N = 257) were randomly allocated to one of four conditions (collaborative implementation intentions; partner-only; implementation intentions; control group) before completing measures at baseline and follow-ups at 1, 3 and 6 months. Outcome measures comprised validated self-report measures of physical activity: the International Physical Activity Questionnaire (IPAQ; Craig et al., 2003) and Self-Report Walking and Exercise Tables (SWET; Prestwich et al., 2012); psychosocial mediators (enjoyment, intention, self-efficacy, social influence); weight and waist size (baseline and 6 months only). Results: As well as losing the most weight, there was evidence that participants in the collaborative implementation-intention group were more physically active than each of the other three groups at 1-, 3- and 6-month follow-ups. Those in the implementation-intention and partner-only conditions did not outperform the control group on most measures. Conclusion: Collaborative implementation intentions represent a potentially useful intervention to change important health behaviors that help reduce weight.