The kinetics of the reactions of isocyanates with thiols in the presence of tertiary amines were examined. The rate of the reaction was of the first order with respect to the concentration of isocyanate, thiol, and tertiary amine. The catalytic action of tertiary amines decreased in the order of triethylenediamine, triethylamine, diethylcyclohexylamine, tributylamine, N-methylmorpholine, pyridine, and quinoline. The relative rate of the reaction of phenylisocyanate with thiols decreased in the order of phenylmethanethiol, 1,4-butanedithiol, 1-butanethiol, 1-dodecanethiol, and thiophenol. The reaction took place more smoothly in strong ionizing solvents. These facts can be explained by the reaction mechanism indicated by the equations [6] to [8].