Dynamic melt viscometry was used to study the kinetics of molar mass decrease in the molten liquid state of a polyurethane elastomer based on 4,4'-diphenyl-methane diisocyanate and 1,4-butanediol. It was found that at constant temperatures chosen between 180 and 220 degreesC the molar mass of the polyurethane melt decreased continuously until a thermal equilibrium was reached. The rates as well as the degree of decrease strongly depend on temperature and time. Based on a kinetic model, the temperature dependence of the zero shear viscosity was shown to be controlled not only by the molecular mobility but also by the change in molar mass. The true activation energy of how (131 kJ/mol) could be calculated after separation of the molar mass contribution. The rate constants and the activation energies of urethane formation and dissociation were also determined. (C) 2001 Published by Elsevier Science Ltd.