The initial rates have been determined for the second-order equilibrium reaction of phenyl isocyanate with a series of phenyl-substituted 1,1-pentamethylene-3-phenyl ureas in the presence of butyltin trichloride catalyst in chlorobenzene at 60°C. These results were compared with the rates of reaction of phenyl isocyanate with 1,1-di-n-butyl-3-phenylurea, 1,3-diphenyl-1-methylurea, N-phenylbenzamide, methyl-N-phenyl carbamate, and 1-butanol. The activation energy for the butyltin trichloride-catalyzed reaction of phenyl isocyanate and 1,1-pentamethylene-3-phenylurea was 12 kcal./mole. At 60°C. this reaction was approximately 11 times as fast as the reverse reaction. The rate of the reaction of phenyl isocyanate and 1,1-pentamethylene-3-phenylurea was directly proportional to the concentration of butyltin trichloride. The activity of this catalyst was 28 times as great as that of triethyltin chloride. Dibutyltin dilaurate, dibutyltin diacetate, dibutylin dichloride, tributyltin chloride, tetramethyltin, trimethyltin bromide, triethyltin fluoride, and 1,2,4-trimethylpiperazine were not catalysts for these reactions. The ureas formed solid complexes with stannic chloride and with butyltin trichloride in which the tin compounds were coordinated through the carbonyl oxygen. A tentative reaction mechanism has been proposed, which involves nucleophilic attack of the urea on the isocyanate through an intermediate ternary complex. The stannic chloride-catalyzed reaction of ureas with isocyanates at room temperature has been shown to have wide application.