UV/H2O2 disinfection of surface water containing humic substances was investigated. UV radiation was applied at doses of 68, 136, 204, 272, 341, 409, 545, 681 mWs/cm(2) to water samples containing various concentrations of humic acid [0-10 rug Dissolved Organic Carbon (DOC)/L] and 0.125 mg/L hydrogen peroxide. Total coliform, numbers were determined after UV radiation and dark incubation of the samples. Results showed that the efficiency of bacterial reduction was reduced significantly with increasing concentrations of humic acid in water. Inactivation coefficients (k) for waters containing 0, 1, 5 ve 10 mg DOC/L humic acid were determined to be 0.0232, 0.0162, 0.0151 ve 0.0135, respectively according to Chick-Watson Model. Results of regrowth experiments indicated that free oxygen radicals produced by H2O2 decomposition were also effective for the inactivation of bacteria after the completion of disinfection process.