Corn milled like flour was crumbled with 5% butter containing a high level of conjugated linoleic acid, then kept exposed to air on an aluminium tray at a layer of 1 cm thickness. Its acid number, peroxide number and fatty acid composition were measured weekly. It was established that during a 24 week long period, there was very little change in the composition of fatty acids, but after this, in parallel with the increasing acid number and peroxide number, the amount of unsaturated fatty acids decreased, while those values for saturated fatty acids did not change considerably. With these investigations, the authors proved the antioxidant effect of conjugated linoleic acid.