Fruit and vegetable (FV) intake was examined among men and women who participated in an online intervention. The psychological constructs involved were outcome expectancies, behavioral intention, planning, and self-efficacy. One purpose of the analyses was the evaluation of a self-efficacy treatment component. The other purpose of the analyses regarded the role of psychological mechanisms that might be responsible for individual differences in the process of behavior change. A two-arm online intervention with a standard and an enhanced intervention group focusing on FV planning was conducted to improve FV intake, followed up at two and four weeks. The intervention groups differed by the additional inclusion of a self-efficacy ingredient in the enhanced intervention. Linear mixed models examined the intervention effects, and a longitudinal structural equation model explored which psychological constructs were associated with changes in FV intake. Participants were N=275 adults of whom n=148 completed the four-week follow-up. Their age range was 18-81 years (Mage=32.50, SDage=14.00). Analyses yielded an overall increase in self-reported FV intake. Moreover, a triple interaction between time, sex, and experimental groups on self-efficacy emerged, indicating that men, independent of treatment conditions, reported an increase in their confidence to improve FV intake, whereas women developed higher FV self-efficacy when being in the enhanced group instead of the standard group. Planning, self-efficacy, and intention mediated between outcome expectancies, and follow-up FV intake. Both intervention arms produced overall improvements in FV intake. The enhanced intervention resulted in a steeper increase in self-efficacy in women compared to men, and compared to the standard intervention. A psychological mechanism transpired that included a sequence leading from initial outcome expectancies via planning, self-efficacy, and intention towards FV intake.