The interacting effects of ascorbic acid and metal ions on carotene oxidation were studied in an aqueous carotene-linoleate solution at pH 7. Ascorbic acid at concentrations up to 10—3 M was a prooxidant. Fe3+ and, to a lesser extent Co2+, acted synergistically with ascorbic acid, the prooxidant effect increasing with metal concentration. Cu2+ formed a prooxidant system with ascorbic acid only at low metal concentration, but as the copper concentration was raised, inversion of activity occurred and the copper-ascorbic acid system exerted a stabilizing action on carotene. Prooxidant effects were enhanced and antioxidant effects weakened in the presence of added lmoleate hydroperoxides. The latter were unstable in the presence of ascorbic acid and especially ascorbic acid + Cu2+. Ascorbic acid itself became unstable in the presence of Cu2+. Oxygen depletion, brought about by the rapid oxidation of ascorbic acid, may be partly responsible for the carotene-stabilizing effect of the Cu*+-ascorbic acid couple. It is postulated that additional stabilization results from the radical-scavenging properties of copper or of a copper chelate formed by ascorbic and/or dehydro-ascorbic acid.