A method of oxidation or pathogen inactivation in water supplies using the combination of treatments of chlorine dioxide and ozone in the sequential steps prior to filtration and distribution. Results indicate that the addition of chlorine dioxide before prezonation reduces the bromate formation and chlorine dioxide can be substituted for preoxidation with ozone, while maintaining the enhanced filtration effects observed from the presence of a preoxidant. A majority of the chlorite formed from reactions of chlorine dioxide with constituents in raw water are converted to chlorate by the following ozonation step.