Allyl isothiocyanate in an aqueous solution was heated and refluxed at 100 degrees C for 1 h. The reaction mixtures were then extracted with methylene chloride and analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The mixtures in aqueous phase were analyzed by high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) equipped with an atmospheric-pressure chemical ionization (APCI) interface. The compounds identified in the methylene chloride extracts included diallyl sulfide, diallyl disulfide, diallyl trisulfide, diallyl tetrasulfide, allyl thiocyanate, 3H-1,2-dithiolene, 2-vinyl-4H-1,3-dithiin, 4H-1,2,3-trithiin, and 5-methyl-1,2,3,4-tetrathiane. N,N'-Diallylthiourea, which was the major degradation product in the aqueous phase from the thermal reaction of allyl isothiocyanate, was identified by using LC-MS (APCI+), direct-probe EI-MS, and (1)H-NMR. The possible mechanism for the formation of these products was proposed.