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The oxidative deterioration of glycerol‐bound polyunsaturated fatty acids (PUFAs) in culinary oils and fats during episodes of heating associated with normal usage (30–90 min at 180°C) has been monitored by high field ¹H NMR spectroscopy. Thermal stressing of PUFA‐rich culinary oils generated high levels of n‐alkanals, trans‐2‐alkenals, alka‐2,4‐dienals and 4‐hydroxy‐trans‐2‐alkenals via decomposition of their conjugated hydroperoxydiene precursors, whereas only low concentrations of selected aldehydes were produced in oils with a low PUFA content, lard and dripping when subjected to the above heating episodes. Samples of repeatedly used, PUFA‐rich culinary oils obtained from restaurants also contained high levels of each class of aldehyde. The dietary, physiological and toxicological ramifications of the results obtained are discussed.