To understand how to better stimulate adult sport engagement, this study investigated effects of gain-framed messages (GFM; Rothman & Updegraff, 2010) on psychological/behavioural outcomes, with or without efficacy-enhancing messages (Latimer et al., 2010). Eligible adults (30-69 years-old) were randomly assigned to one of two experimental conditions (‘GFM alone’ or ‘GFM plus efficacy-enhancing messages’), or a control condition. Participants (N = 232; 62.5% female) completed baseline/screening measures, watched their messaging intervention and reported outcomes one-week later (Time 2), and responded one-month later (Time 3). Comparing the experimental conditions showed non-significant differences for all outcomes, indicating no added benefit of efficacy-enhancing messages. Collapsing the experimental groups and comparing against the control showed significant group-by-time interactions for three ‘outcome expectancies’ (travel, social affiliation, stress relief), with reports higher among experimental participants. Results also indicated that moderate-to-vigorous physical activity significantly decreased at Time 3 within the control, but remained constant among experimental participants.