The aim of the present study was to evaluate responses of potential indicator bacteria (i.e. Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis) to the ultraviolet (UV) radiation and the UV/hydrogen peroxide (H2O2) disinfection processes of surface waters with different qualities in terms of humic content. The UV and the UV/H2O2 processes were applied to waters containing various concentrations of fulvic acid in order to inactivate E. coli, P. aeruginosa and B. subtilis spores. Three fulvic acid (0, 2 and 6 mg l(-1)) and four H2O2 (0, 10, 25 and 50 mg l(-1)) concentrations were used. Results showed that the k values of E. coli, P. aeruginosa and B. subtilis spores varied between 2.22 and 4.00, 1.73 and 3.58, and 1.40 and 1.86, respectively, in all test conditions. The sensitivity of the test organisms followed a decreasing order of E. coli > P. aeruginosa > B. subtilis. Results of the study indicated that the blocking effect of fulvic acid for the UV light was diminished by using H2O2 in combination with the UV radiation. Findings of the present study strongly suggested that the UV/H2O2 process was significantly effective on the inactivation of E. coli and P. aeruginosa in humic waters, whereas it induced little or no apparent contribution to the disinfection efficiency of B. subtilis spores.