The effect of dissociation of hypochlorous acid on the deterioration of black filled and crosslinked ethylene-propylene-diene terpolymer (EPDM) in sodium hypochlorite (NaOCl) solution was studied by laser and electron microscopy observation. Commercial EPDM, containing ethylene (62 %) and diene (4.5 %), was filled with carbon black by the ratio of 60% (w/w). The EPDM plates were immersed in NaOCl solutions of pH 4.5 and 10, at which the free available chlorine (FAC) mainly exists as hypochlorous acid (HOCl) and hypochlorite ion (OCl^-), respectively, for 7 days. The changes in the surface morphology of the sample became remarkable after being immersed in NaOCl solution of pH 4.5 at higher temperature. As a result of immersion in NaOCl solution of pH 4.5, corrosion pits, blisters, and cracks were formed on the surface of the sample, whereas no stickiness on the surface was observed. In addition, NaOCl solution was kept transparent, but there were precipitations of rubber powders of 10-µm order. On the other hand, after being immersed in NaOCl solution of pH 10, stickiness on the surface was observed whereas no blisters and cracks were formed on the surface. Although the NaOCl solution of pH 10 became suspended by carbon black, no precipitation took place. These data indicated that significantly different morphological changes of EPDM in weak acidic and alkaline NaOCl solutions occurred depending on the dissociation of hypochlorous acid.