The present study examined the utility of two forms of measurement of intrinsic motivation in increasing the predictive validity of the theory of planned behaviour. Self-report questionnaires were administered to school pupils (n = 174), University students (n = 129) and adults (n = 157). The data were analysed using confirmatory factor analysis and regression analysis. Confirmatory analysis supported discriminant validity between Forms A and B measures of intrinsic motivation. In addition, hierarchical regression analysis demonstrated that Form B measure of intrinsic motivation increased effectiveness of the theory of planned behaviour in predicting intentions and social behaviour. Further, the regression analysis showed that age and past behaviour did not reduce the effects observed for intrinsic motivation. It is recommended that intrinsic motivation could increase the predictive utility of the theory of planned behaviour. Copyright © 2006 John Wiley & Sons, Ltd.