The influence of temperature, pH, alkalinity, and type and concentration of the dissolved organic matter (DOM) on the rate of ozone (O-3) decomposition, O-3-exposure, . OH-exposure and the ratio R-ct of the concentrations of . OH and O-3 has been studied. For a standardized single ozone dose of 1 mg/L in all experiments, considerable variations in O-3-exposure and . OH-exposure were found. This has important implications for water treatment plants regarding the efficiency of oxidation and disinfection by O-3. In oligotrophic surface waters and groundwaters, minimal calibration experiments are needed to model and control the ozonation process, whereas in eutrophic surface waters more frequent measurements of O-3 kinetics and R-ct values are required to evaluate seasonal variations.