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The kinetics of isothermal cyclotrimerization of bifunctional hexamethylene diisocyanate and trifunctional isocyanurate in the presence of hexabutyldistannyl oxide as a catalyst has been studied with differential scanning calorimetry and dielectric relaxation spectroscopy. It has been demonstrated that a rise in the glass transition temperature in the course of cure of the trifunctional monomer is satisfactorily described by the equilibrium first-order kinetics. The evolution of dielectric parameters is similar for both samples but is characterized by a shift along the time scale because of different specific concentrations of isocyanate groups in the parent monomers. Given the same catalyst concentration, the trimerization of hexamethylene diisocyanate proceeds at a higher rate than that of isocyanurate. For maximally cured systems, this effect manifests itself as a small increase in the glass transition temperature and the shear modulus in the rubbery state and a change in the temperature interval of α relaxation.