Oxidation was measured by oxygen consump-tion in a Warburg apparatus, modified to main-tain constant partial oxygen pressure by auto-matic electrolytic generation of oxygen with automatic recording of the oxygen consumed. The decrease in rate of oxygen consumption on the lowering of partial oxygen pressure at at-mospheric pressure was found to depend on a) the varying influence of the nonoxygen-dependent and the oxygen-dependent reactions of the prop-agation which may vary with the conditions such as the reactivity of the substrate, the tem-perature, and the pH value but which is not affected by light irradiation; b) the varying rate-limiting effect of slow oxygen diffusion, depending on the ratio between the rate of oxidation and the rate of oxygen diffusion. © 1968 American Oil Chemists' Society.