The ability of the theory of planned behaviour to predict objectively measured outcomes was tested in school-age children. Study I (N = 71) showed that the theory of planned behaviour accounted for significant proportions of the variance in intention and that the intentions predicted scores in a science test; study 2 (N = 175) replicated these effects across a broader range of subjects. In addition, study 2 explored the mechanism by which intention was translated into action and demonstrated that intention stability moderated the intention-academic achievement relationship. Moreover, controlling for a range of cognitive and affective variables, intention and positive affect were independent predictors of intention stability. These findings suggest that although positive affect did not influence intention directly, it did influence intention stability, the mechanism by which intentions are translated into action. Future educational interventions should therefore influence both cognitive and affective factors to promote sustained motivation. © 2008 The British Psychological Society.