TiO2 - anatase is a wide bad gap semiconductor with photo-catalytic properties, used in organic dyes photo-degradation. Commercial TiO2 powder (Degussa P25) was used in experiments as dispersions in methyl-orange solutions and as thin films deposited by doctor blade technique. Tests were done in dark (when only adsorption occurs) and under UV irradiation; the efficiency was calculated for different dye concentrations and various ratio solution volume : anatase mass. The adsorption mechanisms are discussed for adsorption and for photo-degradation considering the influence of the catalysts state: dispersed or thin film. The competitive processes adsorption/photo-catalysis are evaluated based on the adsorption parameters. The addition of H2O2 in the photo-degradation systems increases the efficiencies when using thin films, at values comparable to those obtained in dispersed systems.