The ozonation of model systems and several natural waters was examined in bench-scale batch experiments. In addition to measuring the concentration of ozone (O-3), the rate of depletion of an in situ hydroxyl radical probe compound was monitored, thus providing information on the transient steady-state concentration of hydroxyl radicals (. OH). A new parameter, R-ct, representing the ratio of the . OH-exposure to the O-3-exposure was calculated as a function of reaction time. For most waters tested, including pH-buffered model systems and natural waters, R-ct was a constant value for the majority of the reaction. Therefore, R-ct corresponds to the ratio of the . OH concentration to the O-3 concentration in a given water (i.e. R-ct = [. OH]/[O-3]). For a given water source, the degradation of a micropollutant (e.g. atrazine) via O-3 and . OH reaction pathways can be predicted by the O-3 reaction kinetics and R-ct.