TiO2 (anatase) with different microstructure was synthesized by thermal hydrolysis of the titanyl sulfate and studied by X-ray powder diffraction, high resolution transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The effect of titanium dioxide structure, regular or distorted, on the photocatalytic degradation of Acid Orange 7 Dye (AO7) in water upon ultraviolet light was studied. It was found that synthesized TiO2 possesses a relatively high reactivity when illuminated but also show different adsorption in the dark. The relationship between these behaviors depends on the real structure of the catalysts. Catalysts with a perfect structural ordering formed after heating at temperature higher than 500 degrees C show better photocatalytic performance. Small amount of Pt added into the TiO2 structure was found to improve further the catalyst reactivity. Pt-modified Titania catalysts oxidize AO7 more efficiently than P-25 Degussa TiO2. Doping effect of Pt on the structural and photocatalytic properties of the samples is discussed. (C) 2006 Elsevier B.V. All rights reserved.