The curing of an isocyanate (NCO)-ended quasi-prepolymer by reaction with water in stoichiometric ratio was monitored by using differential scanning calorimetry both under isothermal and non-isothermal conditions. A quasi-prepolymer containing 16 wt % free NCO prepared by reacting trifunctional polypropylene glycol (PPG) with polymeric MDl was used in this study. The variation of the effective activation energy with the extent of the curing reaction was calculated by means of model-free differential and integral isoconversional methods. Both isoconversional methods provided similar results showing that the activation energy depends on the extent of the reaction of the quasi-prepolymer with water. The dependence of the effective activation energy proved to be different under isothermal and non-isothermal curing conditions. (c) 2007 Wiley Periodicals, Inc.