Photocatalytic degradation of cationic blue X-GRL (CBX), a strongly adsorbing substrate on TiO2/SiO2, has been investigated by diffuse reflection UV-VIS spectra (UV-visible/DRS) and FT-IR methods. The heterogeneous photocatalysis processes include: adsorption of dye, surface reaction and desorption of final products. The adsorption of CBX onto the surface of TiO2/SiO2 was characterized, and the effect of initial pH of solution on the adsorption was determined. The results demonstrated that CBX was strongly adsorbed at the TiO- site of the catalyst by a penta-heterocyclic-N group. The adsorbed CBX was monitored directly by the UV-VIS/DRS technique during the photodegradation. The results confirmed that the surface photodegradation reaction of CBX was predominantly procedure with the mechanism of direct charge transfer from the semiconductor to the dye adsorbed at TiO. center, and its reaction followed the pseudo-first-order kinetics. This mechanism resulted in the formation of more aliphatic amines and/or amide byproducts, which were further transformed to NH4+. (C) 2003 Elsevier Science B.V. All rights reserved.