In this work, nanocrystalline mesoporous-assembled TiO2 photocatalyst was synthesized by a sol-gel process with the aid of a structure-directing surfactant and employed for the photocatalytic degradation of methyl orange azo dye (monoazo dye), as compared to various commercially available non-mesoporous-assembled TiO2 powders. The experimental results showed that the synthesized mesoporous-assembled TiO2 nanocrystal calcined at 500 degrees C provided superior decolorization and degradation performance to the non-mesoporous-assembled commercial TiO2 powders. In addition, several operational parameters affecting the decolorization and degradation of methyl orange, namely photocatalyst dosage, initial dye concentration, H2O2 concentration, and initial solution pH, were systematically investigated, using the mesoporous-assembled TiO2 nanocrystal. The optimum conditions were a photocatalyst dosage of 7 g/l, an initial dye concentration of 5 mg/l, a H2O2 concentration of 0.5 M, and an initial solution pH of 4.7, exhibiting the highest decolorization rate of methyl orange. (C) 2009 Elsevier B.V. All rights reserved.