The mechanism of the chemical deterioration of black-filled ethylene propylene diene terpolymer (EPDM) in sodium hypochlorite (NaOCl) solution of pH 4.5 was studied at various temperatures and free available chlorine (FAC) concentrations. At pH 4.5, FAC exists predominantly as undissociated hypochlorous acid (HOCl). At relatively low temperatures, HOCl diffused into and reacted with EPDM without changes in appearance of EPDM surface. Morphological changes of EPDM surface, and diffusion and reaction of HOCl were accelerated at elevated temperatures and FAC concentrations. X-ray photoelectron spectroscopy(XPS)analysis showed that C-Cl, C-O, C=O and COO groups were formed on EPDM surface. When the pH of HOCl solution became less than 2.5 after immersion of EPDM, the content of C-Cl group increased markedly, suggesting that chlorine radical might involve in the formation of C-Cl group. These results indicated that chlorination, hydroxylation, oxidization, crosslinking and chain scission reactions occurred between HOCl and EPDM, depending on temperature and FAC concentration. On the basis of the above data, we proposed deterioration processes of EPDM by HOCl.