2-(2'-octenyl)succinic acid has been identified in urine samples from children investigated for a possible inherited metabolic disease. Its structural identification has been achieved by gas chromatography/mass spectrometry using both electron ionization and chemical ionization and by tandem mass spectrometry (MS/MS) using fast-atom bombardment and high-resolution electron-ionization analyses of the molecular ion in a complex biological matrix. The localization of the double bond was obtained by interpretation of a unexpected rearrangement reaction occurring after dimethyl disulfide derivatization.