The damage to tuna fish lipids induced by cooking was investigated in the Thunnus obesus and Th. thynnus varieties, using conventional and fluorescence detection methods, and the results were compared. As a consequence of thermal processing, the peaks at longer wavelengths increased, which correlated with other conventional indices of lipid damage (i.e. carbonyl compound formation, browning and increases in the free fatty acid content). A special significance was given to the fluorescence ratio between the maxima of the excitation emission data at 393/460 nm and 327/415 nm; increases in this ratio as a result of cooking were less dependent on the samples than were other conventional methods of measuring lipid damage.