We ascertained that one of the lachrymator, allyl isothiocyanate (AITC), reacted with alkyl alcohol to form O -alkyl allylthiocarbamate. AITC was incubated with methanol, ethanol (EtOH) or 2-propanol, respectively, and analyzed by gas chromatography/mass spectrometry (GC/MS). On the total ion chromatogram of the reaction mixture, one peak appeared which was different from AITC, and the molecular weight was 131, 145 or 159, respectively. The reaction product of AITC with EtOH was prepared from the mixture. By ¹H nuclear magnetic resonance (NMR) analysis, six pairs of the signals which resembled each other appeared at room temperature and some of the paired signals overlapped by heating. It was suggested that the reaction product of AITC with EtOH was O -ethyl allylthiocarbamate, and that it was composed of the mixture of conformational isomers at room temperature.