The oxidized lubricating naphthenic base oil was used as a stationary phase for the separation of short-chain free fatty acids (SFFA) either as a pure sample or an aqueous solution containing 0.9-1.2 mg/L of each acid. It is found that the oil oxidation for 20 h improved the separation of SFFA in these two sample forms. This separation improvement represents not only the increase in retention volume intervals and peak symmetries in case of the pure sample but also in acid peak areas in case of the aqueous solution sample.