This study investigated the catalytic decomposition of H2O2 by activated carbon (AC) and its implications for degradation of diethyl phthalate (DEP). It was found that AC exhibited excellent catalytic ability for decomposition H2O2 and degradation of DEP. HNO3 modification altered the surface characteristics of AC together with the concentrations and types of AC free radicals (FRs), which further promoted generation of (OH)-O-center dot. Positive correlations were found between FR concentration and generation of (OH)-O-center dot (R-2 = 0.856) and between the proportion of surface-bound hydroxyl groups (COH) and the decomposition rate of H2O2 (R-2 = 0.776), indicating that FRs in AC were the main contributor to (OH)-O-center dot generation, whereas COH groups were predominantly responsible for decomposition of H2O2. Electron capturing studies demonstrated that the decomposition reaction likely involves the transfer of FR electrons to H2O2 to induce formation of (OH)-O-center dot.