AbstractThe 2‐thiobarbituric acid (TBA) test is widely used for measuring rancidity in foods. It is based on the colorimetric determination of a pink chromogen which is formed by reaction between TBA and TBA reactive substances (TBARS). Besides malonaldehyde (MA) many other substances have been reported to react with TBA. The formation of formaldehyde (FA) from the trimethylamine oxide (TMAO) decomposition in sea fish has been extensively studied. Considering that FA reacts with the TBA forming a white precipitate or cloudiness the interference of FA with TBA test is studied concluding that FA contents over 25 μg ml−1extract interfere with the colorimetric determination during the storage of frozen blocks of Patagonian hake (Merluccius hubbsi) filleted and mi