Porous TiO2 and mixed mesoporous(SiO2):TiO2 95:5 in molar ratio) have been synthesized by a sol-gel method followed by a supercritical i-PrOH treatment (300 degreesC 80 bar) and using a template molecule. They have been characterized by X-ray diffraction, scanning electron microscopy and specific surface area measurements. A photocatalytic degradation of phenol and salicylic acid investigated as a function of calcination temperature allows to obtain a best sample at 450 degreesC (SD450 TiO2). The surface hydroxyl amount of the lab-made photocatalyst and that of the commercial powders (Degussa P25, Sachtleben Hombikat UV100 and Acros TiO2) have been compared. This factor is found to strongly modify the 0) adsorption. The commercial Hombikat and Acros TiO2 have been found to be less active than SD450 and P25 TiO2 under identical condition confirming in this way the dominant role of surface OH groups. In addition, for Acros TiO2 the phenol photodegradation shows a formation of numerous oxidative intermediate products. Their identification permits to indicate that the photolysis significantly occurs because of a low concentration of OH. radicals whereas for SD450 or Degussa P25 TiO2 these behaviours appear negligible. (C) 2003 Elsevier B.V. All rights reserved.