This study employed the theory of planned behaviour to investigate the factors underlying intentions to use (time 1, N = 471), and self reported use of alcohol and tobacco (time 2, N = 141) over a 6 month period in a population of students. The TPB provided good predictions of both intentions (alcohol: R = 0.167; attitude, and perceived behavioural control (PBC) significant; tobacco: R = 0.137, PBC significant) and self reported behaviour (alcohol: R = 0.289; tobacco: R = 0.423; intentions and PBC significant for both). Descriptive norms but not moral norms explained additional variance in intentions. Implications for furthering our understanding of these behaviours are discussed.