Although some studies concerning the effect of pH and ozone dosage on TiO2 photocatalysis (O-2/TiO2/UV) have already been published, no complete investigation and explanation of the effects of both parameters on photocatalytic ozonation (O-3/TiO2/UV) have been carried out. Aqueous solutions of neonicotinoid insecticides (thiacloprid and imidacloprid) were chosen as a degradation medium, since they exhibit a high threat for aquatic systems and it is of great importance to find an effective method for their elimination from the environment. In preliminary stability tests, thiacloprid showed higher photo- and chemical stability compared to imidacloprid, therefore its degradation was studied in detail. To assess the suitability of various treatments for degradation and mineralization of thiacloprid in water at different pH values and ozone dosages, we applied ozonation (03) and three different photochemical advanced oxidation processes, namely ozonation, coupled with UV radiation (O-3/UV), O-3/TiO2/UV and O-2/TiO2/UV. Light source emitting mainly in UVA range was applied in all three processes. The photocatalytic ozonation (O-3/TiO2/UV) was found to be the most efficient process irrespective of pH. The synergistic effect of ozone and TiO2 photocatalysis was noticed at acidic and neutral pH, but the synergism was lost at basic pH, probably due to faster self-decompositon of ozone under alkaline conditions. At acidic pH, also the oxidation of chloride anions to chlorate(V) was noticed in O-3/TiO2/UV and in O-3/UV processes. By plotting the disappearance rate constants of thiacloprid degradation in O-3/TiO2/UV and O-3/UV systems as a function of the flow rate of ozone, the synergistic effect of ozone was undoubtedly proven. The slope of the linear fit in case of O-3/TiO2/UV process was considerably steeper than in case of O-3/UV, which would not happen in absence of synergistic effect. The linearity in O-3/TiO2/UV system was lost only at very high flow rates of ozone. (c) 2007 Elsevier B.V. All rights reserved.