AbstractSmall amounts of unsaturated carbonyl compounds, which originate from autoxidized fats, were converted into their corresponding 2,4‐dinitrophenylhydrazones and subsequently partially hydrogenated. Neither hydrazine nor hydrogen gas and platinum oxide gave satisfactory results, but, using palladium on calcium carbonate as catalyst, the 2,4‐dinitrophenylhydrazones could be hydrogenated partially. Only the double bonds of the aliphatic part of the molecule were reduced, whereas the carbon nitrogen double bond and the nitro groups were not attacked. After hydrogenation of 2,4‐dinitrophenylhydrazones, the reaction mixture was analyzed by thin layer chromatography on a Kieselguhr plate impregnated with Carbowax. In this way, the chain length of the unsaturated carbonyl compound, as well as the number of double bonds, could be determined. Carbonyl conjugated double bonds appeared to be hydrogenated faster than isolated double bonds.