The present study examined the moderating role of attitudinal ambivalence within the theory of planned behaviour (TPB). Attitude, subjective norm, perceived behavioural control (PBC), behavioural intentions and self-reported behaviour were assessed in relation to 20 healthy eating behaviours in a prospective questionnaire design in 232 members of the general public in the UK. Between-participants univariate and multivariate analyses indicated attitude-behaviour and PBC-behaviour relationships to be weaker in higher ambivalence compared to lower ambivalence respondents. These effects were confirmed in within-participants analyses. The findings are discussed in terms of the role of ambivalence in furthering our understanding of relationships in the TPB.