in this study, photoanodic chlorine generation and chlorate formation were investigated using a nanoporous TiO2 photo-electrode under an illumination intensity of 7.4 mW/cm(2) UV and biased at +1.0 V vs. SCE. Chlorine formation increased over time finally reaching a steady-state concentration of 42 mg/l in 45 min. On the other hand, chlorate formation was observed after only 5 min irradiation and increased over time. The effect of pH, oxygen concentration and charge separation on chlorine generation and chlorate formation was also investigated. Chlorine production was practically constant over a pH range of 4.0-8.0, but decreased slightly at higher pH values. Chlorate production could be detected at pH values >= 7.0. A significant effect of oxygen concentration on chlorate formation was observed. Chlorate was significantly reduced under nitrogen gas. Chlorine production in the separated photoanode compartment was about 10 fold higher than that in a combined photoanode-cathode system.