In this study, the effects of chloride ion concentration and pH on UV oxidation treatment were examined. Acetone and sodium dodecyl sulfate (ABS) were used as organic substances. The treatment efficiencies of these chemicals by UV/H2O2 oxidation using a laboratory scale UV-free surface reactor (UV-FSR) with or without Cl- addition at different pH values was compared. Results of this study indicated that Cl-concentration and the chemical structure of the substances are more decisive than pH in the oxidation process. There was no AOX at the start of the experiments but as a result of oxidation a de novo synthesis of AOX was observed, and these AOX(de novo) compounds were destroyed during the treatment. Treatment was followed by TOC and AOX measurements. Approximately 98% and 95% TOC removal efficiencies were obtained for the treatment of acetone and ABS containing wastewaters, respectively. (C) 2006 Elsevier B.V. All rights reserved.