Temporal stability of behavioural intentions and perceived behavioural control (PBC) within the Theory of Planned Behaviour were examined as moderators of the cognition-behaviour relationships in two studies. Study 1 (N = 201) examined attendance at health screening appointment (infrequently performed behaviour) using an objective measure of attendance. The impact of intentions and past behaviour on behaviour was moderated by intention stability. In addition, stable intentions were more strongly related to attitudes and past behaviour. Study 2 (N = 407) examined eating a low-fat diet (frequently performed behaviour) using a self-report measure of behaviour. The impact of intentions on behaviour was moderated by intention stability, while the impact of PBC and past behaviour on behaviour was moderated by PBC stability. Stable intentions were more strongly related to attitudes, subjective norms and PBC. Stable PBC was more strongly related to attitudes and past behaviour. The discussion considers the implications of the findings for furthering the understanding of the proximal determinants of behaviour and emphasizes the importance of temporal stability as a key measure of the strength of intentions and PBC.