This study is devoted to a chemical reaction, carried out in a gaseous phase under isothermal conditions and without internal convective flow. Traditionally, in the mass-balance equations, the transport of reactants and products in a porous catalyst is introduced by the diffusion phenomenon in gaseous phase. The reaction rate then is written as a function of the reactant's and product's concentrations in gaseous phase [G.F. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design, Wiley, New York, 1979]. However, in stationary conditions and with the hypothesis of the thermodynamic equilibrium between the gaseous and the adsorbed phase, the existence of a concentration gradient in the gaseous phase brings about a concentration gradient in the adsorbed phase. The diffusion process occurring in the adsorbed phase is usually neglected in the traditional modeling. The present work aims to investigate the influence of the considered diffusion processes on the evaluation of the performance of the catalyzed chemical reaction. The analysis is based on the study of a simple chemical reaction and consecutive reactions. The study shows that the flux of consumed reactant, evaluated by a traditional way could be different from the flux obtained with the diffusion in the adsorbed phase. Also, in these conditions the kinetic parameters obtained by the traditional modeling of the experimental results do not represent then the real kinetic parameters. (c) 2006 Elsevier B.V. All rights reserved.