The isotherm of adsorption and the kinetic of photocatalytic degradation of two azo dyes, Remazol Black 5 (RB5) and Procion Red MX-5B are reported and compared using different concentrations of dyes. The photocatalytic degradation is performed in aqueous solutions in the presence of TiO2 Degussa P25 under artificial UV light radiation (lambda > 290 nm). The isotherms of adsorption of the both dyes are very different. These differences are discussed considering the pH of the solution and the structure of the dye. In opposition to the isotherm of adsorption, the kinetic of both dyes are very similar, and fit well with the Langmuir-Hinshelwood model in the concentration range 5-60 mu mol/L. However, at higher dyes concentration, the initial rate of decolorization decreases. The important absorption of light by the dyes was suggested to be at the origin of this decrease of the decolorization. The absence of correlation between isotherm of adsorption in the dark and the decolorization kinetics is discussed. The attribution of main UV-vis bands of each dye has been performed by molecular modelisation using Density Functional Theory (DFTT). Their evolution as a function of time, the evolution of pH during the photocatalytic process and the determination of the decrease of Total Organic Carbon (TOC) and of the evolution of inorganic ions are used to better understand the decolorization mechanism occurring for these both dyes. (C) 2007 Elsevier B.V. All rights reserved.