thermal aging of hydroxy-terminated polybutadiene/isophorone diisocyanate (htpb/ipdi)-based polyurethane was studied as a function of nco/oh ratio (1.0 and 0.85). samples were aged in air ovens at 50, 60, 65, and 70 oc for periods of time from 1 to 34 weeks. changes in chemical (swelling testing and ft-ir spectroscopy) and mechanical (tension testing and hardness) properties were evaluated throughout the aging assays. correlation between equilibrium swelling ratio and elastic modulus, coupled with changes in ft-ir spectra, indicated oxidative cross-linking as the dominant mechanism for both molar ratios investigated. however, determination of arrhenius activation energy resulted in values of (82 ± 10) kj.mol-1 and (156 ± 30) kj.mol-1 for 1.0 and 0.85 nco/oh ratios, respectively, thus revealing faster oxidative degradation kinetics for higher urethane linkage networks.