A randomised control trial evaluated the effectiveness of a theory-based persuasive leaflet designed to encourage students to undertake at least one additional physical exercise session a week. Participants were 503 secondary school students attending a school in South-East England. The leaflet was written to target potentially modifiable cognitive antecedents of exercise specified by the Theory of Planned Behaviour. It was separately augmented with two cognitive change techniques, resulting in three intervention conditions, leaflet alone; leaflet plus motivational quiz, and leaflet plus implementation intention prompt, as well as a no-leaflet control condition. Cognitions and behaviour were measured immediately before and 3 weeks after intervention. The results showed that all three-leaflet interventions significantly increased reported exercise, intention to exercise and related cognitions, compared to the control condition, but did not differ in their impact. Mediation analysis showed that intervention effects on exercise were partially mediated by intentions and perceived behavioural control.