Treatment of 2-chlorophenol (2-CP), 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) in aqueous solutions with direct photolysis (254 nm), ozonation and ozonation with photolysis was studied. A model was developed to simulate chlorophenol treatment in a semibatch column using these processes under acidic conditions in which the chemical reactions are slow. Satisfactory results were obtained in simulating the chlorophenol and ozone concentrations, and the concentrations of an aromatic quinone, hydrogen peroxide and chloride ions formed in the course of the processes. Chlorophenol oxidation systems, however, appeared to be very complex and the formation of intermediate and final products were dependent on the reaction conditions. The degradation rate of chlorophenols was not enhanced by the combination of ozone with UV radiation compared with ozonation alone. Oxidation at low pH in both processes proceeded mainly through reactions with molecular ozone.