The TiO2-mediated photocatalysis process was used to successfully degrade dye pollutants. To better understand the mechanistic details of this TiO2-assisted photodegradation of the Victoria Blue R (VBR) dye with UV irradiation, forty-four intermediates of the process were separated, identified and characterized by HPLC-PDA-ESI-MS (high performance liquid chromatography-photodiode array-electros pray ionization-mass spectrometry) technique in this study, and their evolution during the photocatalytic reaction is presented. The results indicated that the N-de-alkylation degradation of VBR dye took place in a stepwise manner to yield mono-, di-, tri-, tetra-, penta-, hexa-N-de-methylated and mono-N-de-ethylated VBR species, with N-hydroxyalkylated intermediates generated during the process. Moreover, the oxidative degradation yielded 4-diethylaminophenol, 4-diethylamino-4'-diethylaminobenzophenone, 4-(N,N-dimethylaminophenyl)-4'-(N'-ethylaminonaphthyl)ketone, 4-(N-ethylamino)naphthenol and their N-de-alkylated products. The reaction mechanisms of TiO2/UV proposed in this study should be useful for future application of the technology to the degradation of dyes. (C) 2008 Elsevier B.V. All rights reserved.