Cyclotrimerization of phenyl isocyanate at 30-50-degrees-C in the presence of 1-butanol or butyl-N-phenylurethane and catalysts of tertiary amine, quaternary ammonium salt, or carboxylate types in 1,4-dioxane was studied by reverse-phase liquid chromatography. Urethane, allophanate, urea, biuret, isocyanurate, and isocyanate model compounds were found in the system. Stability of all reaction products was also examined. The time dependence of evolution of various products depends on the catalyst type and temperature. The overall reaction scheme was proposed.