An unknown compound, observed as a prominent peak in extracts of Rocket salad (Eruca sativa) that were analyzed for pesticide residues by gas chromatography with the electron capture detection (GC-ECD), was isolated by high-performance liquid chromatography (HPLC). The compound was identified by gas chromatography/mass spectrometry (GC/MS), Fourier transform-infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR) as bis(4-isothiocyanatobutyl) disulfide. For the final structural confirmation, the compound was synthesized. Its formation by the oxidation of 4-(mercapto)butyl isothiocyanate, a glucosinolate breakdown product, was elucidated.