The formation of surface modification on TiO2, which can be easily attained by a simple addition of ascorbic acid to aqueous ToO(2) suspensions, uniquely affects both photocatalytic reactions and mechanisms. Surface modification results in the formation of a colored surface complex that causes red shift of the absorption threshold of TiO2, enhancing utilization of the solar spectrum. The decolorization rate of methyl orange, taken as a model for recalcitrant azo-dye, was observed to be the function of surface modified TiO2 used during the process. It was found that surface modified TiO2 was more effective for oxidation of azo bond than pure TiO2 irradiated by both UV light and solar light. Through investigating the effects of dissolved oxygen, CCl4 and tert-butyl alcohol on the decolorization of methyl orange, the mechanisms are proposed that the superoxides (O-center dot(2)-) mediated oxidation pathways are dominant for cleavage of azo bond of methyl orange on surface modified TiO2 with ascorbic acid, whereas the OH radical mediated oxidation pathways are largely inhibited due to the hindered complexation of ascorbic acid on the surface of TiO2. Finally, elementary charge transfer process on surface modified TiO2 and the implication to photocatalytic reaction pathway are discussed. (c) 2005 Elsevier B.V. All rights reserved.