Hydroxy-terminated polybutadiene (HTPB) prepolymer prepared by free-radical mechanism at Chemical Engineering Complex, Vikram Sarabhai Space Center, Thiruvananthapuram was reacted with toluene diisocyanate (TDI), a curative at varying stoichiometric ratios (r = [NCO]/[OH]) equal to 0.7, 0.8, 0.9, 1.0, and 1.1 at a constant temperature of 70degreesC. The increased rate of viscosity of polyurethane networks formed was used to calculate the rate constant. Similarly, the rate of change of viscosity of curing networks at different 7, values (0.7-1.1) and temperature intervals (30-70degreesC) was used to calculate activation energy. The results of the curing networks showed that the rate constant at r > 0.9 is constant and the rate of decrease of activation energy is lower at r less than or equal to 1; thereafter, it becomes quite significant.