The influence of water quality parameters (dissolved organic matter and alkalinity) on the efficiency of the ozone/hydrogen peroxide (O-3/H2O2) advanced oxidation process (AOP) relative to the corresponding conventional ozonation process was investigated. In natural waters with a high natural organic matter content (greater than or equal to3 mg/L), O-3 decomposition is controlled by radical-type chain reactions. Thus, the overall OH radical oxidation capacity is nearly independent if conventional ozonation is replaced by O-3/H2O2 AOP. In contrast, when the organic matter content in the natural water is low (less than or equal to1 mg/L), the addition of H2O2 considerably enhances the oxidation capacity by OH radicals. A constant ratio between the OH radical and O-3 concentration (R-ct) was found during both treatments, making it possible to predict the oxidation of micropollutants with O-3 and OH radicals if the rate constants of both oxidants are known. The oxidation of atrazine during ozonation and the combined process O-3/H2O2 could be accurately predicted in natural waters.