ObjectiveThis study investigated the combined influence of social structural factors (e.g. income) and cognitions in predicting changes in physical activity. Four models were tested: (a) direct effects (social structural factors influence behaviour controlling for cognitions), (b) mediation (cognitions mediate social structural influence), (c) moderation (social structural factors moderate cognition–behaviour relations), and (d) mediated moderation (cognitions mediate the moderating effects of social structural position).DesignBaseline and 3‐month follow‐up surveys.MethodsA random sample of 1,483 adults completed self‐report measures of physical activity at baseline and 3‐month follow‐up. Measures of age, gender, education, income, material and social deprivation, intention, perceived behavioural control (PBC), and intention stability also were taken.ResultsApart from age, social structural factors exhibited very small or marginal effects on behaviour change, and only education moderated the intention–behaviour relation. In contrast, the magnitude of direct effects of the social cognition variables was comparatively large and intention stability mediated the moderating effect of education.ConclusionsStable intentions and PBC are the key predictors of changes in physical activity. Consequently, our findings would suggest the value of focusing on cognitions rather than social structural variables when modelling the determinants of phys