The change in tensile strength of black-filled ethylene propylene diene terpolymer (EPDM) in sodium hypochlorite (NaOCl) solution of pH 4.5 was studied. At pH 4.5, free available chlorine mainly exists as undissociated hypochlorous acid (HOCl), which can diffuse into EPDM and cause the occurrence of corrosion pits, blisters, cracks, and precipitations of rubber powders. Analysis of transverse relaxation measured by ¹H pulsed-NMR showed that the immobilized rubber phase increased after being immersed in NaOCl solution at higher temperatures, at which surface morphological change became remarkable. This indicates the hardening of EPDM and the formation of brittle surface layer. However, the decrease in tensile strength occurred regardless of the detection of the brittle layer. The diffusion behaviors of Cl and O atoms were investigated by electron probe micro analysis. At low temperatures close to the freezing point, only Cl atoms were diffused into EPDM, whereas no significant decrease in tensile strength was observed. The tensile strength of EPDM decreased with the increase in the depth of diffusion of O into EPDM. These results indicate that during elongation process of EPDM specimen, the cracking which occurs in the surface-oxidized layer acts as a notch of the specimen. It appears that the cracks formed in the brittle layer cannot develop into the inner EPDM.