Methyl linoleate was oxidized in model systems consisting of either cellulose or casein with which the lipid was dispersed with water containing cobalt salts. The dispersion was extruded into Warburg flasks, frozen and freeze-dried at 100 μ Hg and with platen temperatures of 80 F. The samples were then humidified over saturated salt solutions to give moisture contents from less than 1 g H2O/100 g solids up to 30 g H2O/100 g solids. The higher moisture contents were obtained by addition of glycerol to the model system during preparation and humidification at 60-75% RH. Chelating agents including EDTA and citric acid in concentrations of 1 to 10 moles per mole of cobalt ion were used in some experiments. Oxidation was followed manometrically and by peroxide analysis. At low water contents, water acts as an antioxidant through hydration of metallic catalysts. As the moisture content increases, water promotes oxidation through its solvent activity. In the region of capillary condensation, antioxidant effect of metal hydration is overshadowed by the prooxidant effect of metal solubilization. The water soluble chelating agents such as EDTA act on metals in aqueous solution and their activity is promoted by increased moisture content.