Dicarboxylic aciduria caused by enhanced or inhibited fatty acid metabolism is usually described as increased urinary excretion of saturated medium-chain dicarboxylic acids, such as adipic, suberic, and sebacic acids. Besides these saturated acids, increased excretion of unsaturated dicarboxylic acids is also observed. However, the structural identities of these unsaturated dicarboxylic acids are largely unknown. Using synthetic authentic samples, dual capillary column gas-liquid chromatography, and capillary column gas-liquid chromatography-mass spectrometry, we have identified these acids as trans-2-hexenedioic, trans-3-hexenedioic, cis-3-octenedioic, cis-4-octenedioic, and trans-3-octenedioic acids. The mass fragmentation pathways of these compounds (as trimethylsilyl derivative) are described. We speculate that the metabolic origin of cis-3-octenedioic acid is oleic acid through the intermediate cis-5-decenedioic acid. We also propose that cis-4-octenedioic acid is derived from linoleic acid. The metabolic origin of trans-3-octenedioic acid is less certain. Since no corresponding cis-hexenedioic acid could be detected in urine, the metabolic degradation of unsaturated dicarboxylic acids appears to terminate at octenedioic acid. Trans-2-hexenedioic acid is probably derived from dehydrogenation of adipic acid.