Most of the chromogen formed when peroxidized material is heated with thiobarbituric acid (TBA) can be ascribed to a colored complex formed between malondialdehyde (MDA) and TBA. Even when little MDA is present, large amounts of MDA-TBA adduct can be formed. This is because lipid peroxides break down to release MDA during the test conditions. Iron is not essential for the breakdown of the peroxides but is essential for the formation of TBA reactivity. This can be related to the ability of iron to decompose lipid peroxides with the release of peroxy radicals, which are precursors of MDA. These peroxy radicals, when released, can initiate further peroxidation during the heating stage of the TBA test. Fatty acids in the absence of lipid peroxides do not undergo significant peroxidation during the acid-heating stage of the TBA test.