Oxidation and oligomeric products of sunflower oil were determined by gel permeation chromatography after transformation into methyl esters and prefractionation by urea. During oil refining the content of oxidation monomeric products increased from 0.9% to 1.5%, the dimer content from 0.06% to 0.32%, and the trimer content from 0.02% to 0.10%. During hydrogenation the content of polar monomeric oxidation products decreased to 0.9%, the dimer content to 0.28% and that of trimers to 0.03%. Oligomers were partly adsorbed on the surface of catalyst and carrier, because fat isolated from the catalyst contained more than 3% of oligomers. The content of oligomers established in the refined sunflower oil and in the hydrogenated sunflower oil produced on an industrial scale was lower than values given as limiting with respect to hygiene safety, sensory value and stability against oxidative rancidity.