Abstract Lake water with a high content of dissolved organic matter (DOM) was pretreated with ozone, ultraviolet (UV) radiation, and a combination of ozone/ultraviolet radiation (UV/O 3) before postchlorination. The chlorine consumption rate, the light absorbance (at 254 nm), the trihalomethane formation potential (THMFP), the mutagenic activity (as detected by the Ames test), and the concentration of the strong bacterial mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) were determined. The ability of the combined UV/O 3 treatment to destroy mutagen and THM precursors in the water was dependent on the dose applied. Low UV/O 3 doses seemed to produce new molecular sites in the DOM that, compared to those in ozonated water, enhanced the chlorine consumption rate, the THMFP, and the formation of mutagenic compounds. When higher UV/O 3 doses were applied, these and the original precursor sites were destroyed more efficiently than by using ozone alone. The precursors to MX were destroyed both by O 3 and by UV/O 3 in a dose-dependent manner, while UV irradiation alone resulted in a slight increase in the mutagen (including MX) precursor levels.