The fate of melamine has been studied under UV light irradiation (medium pressure mercury arc) in the presence of TiO2 and H2O2. The increase in the concentration of aromatic and aliphatic photo-products were determined in solution concomitantly with the decrease observed for the melamine concentration. The main intermediate products occurring during photodegradation were identified by liquid chromatography coupled mass spectrometry (LC-MS) with the MS detection in positive and negative ions. The oxidation of melamine under UV-irradiation in the presence of H2O2 proceeds step by step leading to ammeline, ammelide, and finally to cyanuric acid. The mineralization of melamine was not observed due to the formation of cyanuric acid as intermediate photo-product. The adsorption of melamine on TiO2 was found to vary with the pH of the suspension increasing at pH > 5 due to the charge-exchange transfer between the adsorbent and the adsorbate. Two different bioassays using Vibrio fischeri and Daphnia magna have been used to test the evolution of toxicity during the UV/TiO2/H2O2 treatment. The toxicity of the initial melamine solution seems to increase due to the intermediates generated in solution. (C) 2004 Elsevier B.V. All rights reserved.