The present study tests the efficacy of a single implementation intention to increase intake over a 6-month period, and investigates whether deploying a second implementation intention at 3 months can sustain the long-term impact, compared with passive and active control groups. Participants (N = 650) completed single-item and FFQ measures of behaviour and motivation at baseline before being randomised to one of six conditions in a between-persons design. Intention to treat analysis revealed that for the single-item measure: (1) a single implementation intention is an effective means of initiating fruit and vegetable increase over a 3-month period, but this effect is not sustained over 6 months; (2) administering a second implementation intention at 3 months was successful in increasing intake over 6 months, and acted as a booster on the initial impact; and (3) neither the passive nor active control condition had any impact on behaviour. However, no effects of the manipulations on fruit and vegetable intake were found when behaviour was assessed by the FFQ measure. Secondary analyses showed that reported increases in intake were not related to demand characteristics. The findings are discussed in relation to their conceptual and practical value.