Changes in the physical and mechanical behavior induced by two sterilization methods, gamma irradiation and ethylene oxide, were determined on two commercial medical-grade segmented polyurethanes. The two materials have different chemical composition : one is an aromatic poly(ether urethane urea), Biospan(TM), and the other an aliphatic ether-free polyurethane, Chronoflex(TM). Properties before and after sterilization procedures were compared resulting in specific structural changes for each formulation. The thermal and mechanical properties were examined using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), stress-strain measurements, and its hysteresis cycle. Molecular weight measurements were performed by gel permeation chromatography (GPC). Sterilized Biospan samples showed a decrease in the soft-segment glass transition temperature (T-g,T-s) and an increase in the soft. segment crystallization heat along the quenching process. Sterilized Chronoflex materials showed the opposite behavior. The hysteresis percent and residual strain percent increased after sterilization. The same effect was observed when irradiation dose and strain level increased. Surface analysis performed by scanning electron microscopy showed magnification of original surface defects after sterilization.