A new colorimetric method for microdetermination of isothiocyanates was established, using 3,6-dinitrophthalic acid as a color developing agent in a potassium carbonatesodium thiosulfate solution. The reaction conditions were examined with allyl isothiocyanate as a model compound. The outline of the procedure is as follows : To 2.0 ml of allyl isothiocyanate solution (10-150 μg/ml), (dissolved in 50% ethylene glycol solution, 1.0 ml of 3,6-dinitrophthalic acid monopyridinium salt solution (0.1%), and 1.0 ml of alkaline solution (40% potassium carbonate and 30% sodium thiosulfate) are added and mixed well. The mixture is heated in a boiling water bath for exactly 10 minutes, cooled in running water, and diluted to 20.0 ml with water. The absorbance of the developed color is read at 400 mμ against the reagent blank. In this color reaction, 3,6-dinitrophthalic acid was assumed to be reduced to 3-amino-6-nitrophthalic acid by S^<2->, which was liberated from allyl isothiocyanate in the alkaline medium.