Earlier studies on lipid oxidation at low oxygen pressure were continued to cover the effect of low oxygen pressure in the presence of added metal salts. Addition of Cu2+ and Fe3+ salts was shown to exert a catalytic effect, which, on increasing metal concentration, may reach a maximum and then decrease. This so called conversion is favored at low oxygen pressure (e.g., corresponding to 1% O2 at atmospheric pressure). Under certain conditions it may result in an antioxidative effect. The phenomenon is more pronounced at pH 5 than at pH 7, and it is dependent on the anions present and the ratio between the concentration of the catalyst and that of the substrate. A tentative explanation of these effects of Cu2+ and Fe3+ salts at low oxygen pressure, which are not given by Fe2+ and Co2+ salts, is offered. © 1971 AOCS.