Measurements of tensile and tearing resistance have been conducted as a function of aging time and aging temperature for a polychloroprene (CR). The steady increase in stiffness indicates that crosslinking is dominant during the aging of CR. In the early stage of aging, tensile strength and tearing energy increase thanks to an optimized balance between the strength enhancement from the crosslink network and the network capability in dissipating energy. Prolonged aging after a characteristic time results in a gradual decrease in strength and tearing energy and this change is more pronounced with higher aging temperatures. The superposition principle between aging time and aging temperature was used to determine the activation energy controlling the change of tearing energy in thermal aging. This value agrees with the activation energy E-a = 91 kJ/mol reported in current literature by monitoring the oxygen consumption rates during the aging of CR, suggesting that the change in tearing energy is controlled by chemical reactions much the same as the change in oxygen consumption dissolved in the material. It was also found that the variation in the strain energy density to break is also controlled by the same activation energy. (c) 2005 Springer Science + Business Media, Inc.