The objective of this study was to investigate the oxidation of synthetic organics by ozone (O3) and ozone in combination with ultraviolet light (O3/UV) under different water quality conditions in a laboratory set-up. Carbonate and bicarbonate ions (alkalinity) retarded the efficiency of nitrobenzene removal in both processes. The alkalinity effect was much more pronounced in the ozonation process than in the O3/UV process. For the O3/UV process, increasing the pH reduced the removal rate of nitrobenzene, although for the ozonation process an increase in the removal rate is expected. In the O3/UV process, the rate of organic removal can be first or zero order, depending upon the concentration of the target compound and the alkalinity. The kinetic model presented in this paper can explain the experimental observations related to the effect of pH, alkalinity, and initial nitrobenzene concentration on the removal rate of nitrobenzene.