AbstractIt was demonstrated that bothgamma anddelta saturated lactones are present in highly peroxidized vegetable oils. In the oils which were investigated thegamma isomers are predominant. Additional lactones also form when the hydroperoxides are reduced. Although no lactones were detectable in fresh, refined soybean oil, considerable amounts of bothgamma anddelta lactones were found to be present in highly peroxidized samples of cottonseed and soybean oils.The lactones in the peroxidized oils were concentrated by column chromatography on silica gel and by vacuum distillation. Gas‐liquid chromatography was used for separation and identification. This has been supplemented by thin‐layer chromatography, infrared spectrophotometry, and nuclear‐magnetic‐resonance spectrometry.