The research tested the efficacy of partner- and planning-based interventions to reduce dietary fat intake over a 6-month period. Randomized controlled, blinded, parallel trial. A computer randomization feature was used to allocate council employees (N = 427, of which 393 completed baseline measures) to one of four conditions (partner + implementation intentions, partner-only, implementation intentions, and control group) before they completed measures at baseline and follow-ups at 1, 3, and 6 months post-baseline. Outcome measures were comprised of validated self-report measures of dietary fat intake (saturated fat intake, fat intake, ratio of good fats to bad fats); psychosocial mediators (enjoyment, intention, self-efficacy, social influence, partner support); weight and waist size (baseline and 6 months only). Data from 393 participants were analysed in accordance with intention-to-treat analyses. All intervention groups reported greater reductions in fat intake than the control group at 3 months. The partner-based groups increased the ratio of good fats to bad fats at 3 and 6 months and lost more inches on their waist, versus the non-partner groups. The impacts of the partner-based manipulations on outcomes were partially mediated by greater perceived social influences, partner support, and enjoyment of avoiding high-fat foods. The partner-based interventions also increased intention and self-efficacy. However, the effects in this study were typically small and generally marginally significant. Partner-based interventions had some positive benefits on dietary-related outcomes at 3 and 6 months. Support for implementation intentions was more limited.