AbstractA spray reagent of 4‐amino‐5‐hydrazino‐1,2,4‐triazole‐3‐thiol, 2% in aqueous 1N NaOH, was used for the detection of aldehydes on chromatograms of lipids. Purple spots are produced with aldehydes, and 1 μg of palmitaldehyde can be detected easily on thin layer chromatograms. The reagent is specific for aldehydes and does not react with ketones. The aldehyde moiety from plasmalogens is detected only after acidic hydrolysis. Purple or red stains arise also with ozonides. Simple precautions against autoxidation of the lipids should be taken to prevent formation of aldehydes which could lead to misinterpretations.