This study has examined the bactericidal effect of chlorine dioxide in untreated artificial and domestic wastewaters and secondary effluent of various organic loads. Results indicated that the inactivation of Escherichia coli in artificial wastewater was similar with that in real municipal wastewater. Among three waters, the bactericidal effect of chlorine dioxide was lowest in secondary effluent. The bacteria log inactivation increased by up to threefold when the COD concentration of raw wastewater was decreased by half. An unfavorable COD effect was also observed for the disinfection of secondary effluent. To explain the COD effect on bacteria inactivation, chlorine dioxide residuals were measured with time through each disinfection process. Results from statistical analyses have revealed that, in comparison to the correlations using CT values, the inactivation data can be better correlated with the ratio of COD to ClO2 concentrations. The results of this study would be a useful guide for many municipalities and communities in determining chlorine dioxide dosages for water and wastewater disinfection systems. (C) 2009 Elsevier B.V. All rights reserved.