Recent research has shown that action planning (AP) for physical activity is a behaviour in itself and has its' own psychosocial variables. Guided by self-efficacy theory (SET), the purpose of this study was to examine the mediating role of outcome expectations on the relationship between individuals' change in self-efficacy and AP. Inactive individuals (N = 177, 57% white, 56% males, Mage = 29.7 [SD=9.92]), who had the intention to increase their physical activity in the next 6 months, answered an online questionnaire. They read one of two randomized AP messages (i.e., gain or loss framed) and were asked about their self-efficacy pre- and post-message, their outcome expectations post-message, and given an option to create an action plan. No group differences were found on AP [X 2 (1, N = 177) = 0.12, p = .73]; therefore, the message groups were collapsed for this analysis. Residual changes in self-efficacy predicted outcome expectations post-message [b = 0.62, 95% confidence intervals (CI): 0.45, 0.78] and outcome expectations predicted AP [b = 0.40, 95%CI: 0.07, 0.72]. The significant indirect effect showed that outcome expectations mediated the relationship between change in self-efficacy and AP behaviour [B = 0.24, 95% CI: 0.05, 0.49] as per SET. SET can therefore help further our understanding of how to promote AP in inactive adults. Future interventions aimed at improving AP could focus on fostering AP specific psychosocial variables such as those in SET.