The aim of this study was to investigate the effect of an antioxidant supplement on training induced changes in VO₂max in well-trained subjects. In a double-blinded placebo controlled design, a total of 40 amateur soccer players and 14 multi sports athletes were block-randomised into a placebo group or an antioxidant supplemented group (SUP). The SUP group consumed two commercially available antioxidant/vitamin supplements for 6 weeks according to manufacturer instructions (LifePak® Essentials Super A) and the placebo group took a similar amount of placebo tablets. Before and after the supplementation period, the athletes were tested for skin carotenoid score and VO₂max. During the supplementation period all subjects continued their normal training in the preparation phase of the season. Six week supplementation with the antioxidant vitamin/mineral supplement increased skin carotenoid score by 27 ± 6% while no change was observed in the placebo group. VO₂max increased by 5.6 ± 2.0% in the placebo group while no change was observed in the SUP group. The relative increase in VO₂max was significantly higher in the placebo group than in the SUP group. In conclusion, the antioxidant vitamin/mineral supplement used in this study seemed to reduce training efficiency indicated by the lack of increase in VO₂max in the SUP group.