Methyl linoleate and water containing heme, ferrous sulfate, cysteine, or ascorbic acid were shaken, and the oxidation was measured by means of a Warburg manometer. In spite of a small area of the interface between the lipid and water, oxidation was greatly accelerated by these catalysts. The catalyses occurred limitedly in the interface region, except for cysteine. Triphenylphosphine added to the lipid phase inhibited the catalyses completely. A small amount of Tween 20 (0.2%) almost completely inhibited these catalyses, with the exception of that by cysteine. It is suggested that trace amounts of some active peroxides react with the catalysts and supply radicals on the interface.