Upon chemical, radiation-induced or enzymatic oxidation, cis-polyunsaturated fatty acids, i.e., C18:2(n-6), C18:3(n-3), C20:2(n-6), C20:3(n-6), C20:3(n-3), C20:4(n-6), C20:5(n-3), C22:2(n-3), C22:4(n-6), C22:6(n-3), were found to generate saturated short and medium-chain length dicarboxylic acids, which can be regarded as a distinctive feature of the particular double bonds positions in the polyunsaturated fatty acid molecule. Two different dicarboxylic acids, which were unambiguously quantified by GC-MS, were produced from a single fatty acid: one deriving from the oxidative splitting at the level of the first double bond in the molecule, the other being two-carbon-atoms lower homologous. Formation of dicarboxylic acids occurred also from triacylglycerols and phospholipids containing cis-polyunsaturated fatty acids. In this case, following oxidation, the diacids remained covalently bound to the starting molecule and transesterification was necessary for identification. Being extremely stable and easily detectable compounds, dicarboxylic acids may be considered potential markers of oxidative attack to both free and esterified unsaturated fatty acids.