The way that incidental affect impacts attitude change brought about by controlled processes has so far been examined when the incidental affective state is generated after dissonance state induction. We therefore investigated attitude change when the incidental mood occurs prior to dissonance state induction. We expected a negative mood to induce systematic processing, and a positive mood to induce heuristic processing. Given that both systematic processing and attitude change are cognitively costly, we expected participants who experienced the dissonance state in a negative mood to have insufficient resources to allocate to attitude change. In our experiment, after mood induction (negative, neutral or positive), participants were divided into low-dissonance and high-dissonance groups. They then wrote a counterattitudinal essay. Analysis of their attitudes towards the essay topic indicated that attitude change did not occur in the negative incidental mood condition. Moreover, written productivity–one indicator of cognitive resource allocation–varied according to the type of incidental mood, and only predicted attitude change in the high-dissonance group. Our results suggest that incidental mood before dissonance induction influences the style of information processing and, by so doing, affects the extent of attitude change.