The accurate description of NH3 adsorption/desorption from the catalyst surface is the basis for the correct description of the NH3-SCR catalytic chemistry. Currently, Temkin-type kinetics are widely and successfully used to describe the NH3 adsorption/desorption process over V-based and metal-promoted zeolite catalysts in the 200-600 degrees C T-window: in this approach the heterogeneity of the catalytic surface is taken into account considering the activation energy of the desorption process as a linear function of the adsorbed species surface coverage. In the present work, carried out over a commercial Fe-zeolite SCR catalytic system, we show that this modelling approach fails when extended to lower temperatures (T <= 150 degrees C) and in the presence of water, conditions of growing interest for Diesel exhaust after-treatment systems. Based on literature evidence, an improved, physically consistent dual-site modelling approach is implemented, resulting in an accurate description of the ammonia adsorption/desorption process in the extended 50-550 degrees C temperature window. (C) 2011 Elsevier B.V. All rights reserved.