The changes in tensile properties of black-filled ethylene propylene diene terpolymer (EPDM) in sodium chlorite (NaClO₂) solution and chlorine dioxide solution (ClO₂) were studied. The diffusion behaviors of Cl and O atoms were investigated by electron probe micro analysis. The deteriorated layer in which Cl and O atoms were diffused was formed on the surface of EPDM immersed in the NaClO₂ solution and the ClO₂ solution. Dissociated chlorite ion (ClO₂^-) did not diffuse into EPDM, but ClO₂ did and undissociated chlorite acid (HClO₂) possibly did. It was suggested that chlorine dioxide diffuses into EPDM faster than hypochlorous acid (NaClO). The tensile properties strongly depended on the depth of diffusion of O (D_O) into EPDM, regardless of the type of solution. The hardening of the deteriorated layer enhanced the stress at 100% strain (S₁₀₀), and the cracks generated in the deteriorated layer resulted in the decrease in the tensile strength (T_B) and the elongation at break (E_B). The relationship between T_B and D_O was similar to that between T_B and the length of the physical notch given by the razor. The thickness of the degraded layer was found to be equivalent to the physical notch.