Ag/Al2O3 catalysts synthesized by the common impregnation technique with various Ag loadings were characterized by the NOx-Temperature-Programmed Desorption (TPD) method and their catalytic performances were evaluated in the selective catalytic reduction of NOx by propene (C3H6-SCR). It was found that the NOx uptake decreased linearly as the Ag surface density increased up to about View the MathML source and then leveled off. This behavior was attributed to the formation of a pseudo monolayer of Ag2O clusters, i.e. to the maximum loading of silver per unit surface area of Al2O3View the MathML source for which highly dispersed Ag2O clusters are preserved on freshly calcined samples. It was also found that this optimum Ag surface density could be correlated to the catalytic performances in the C3H6-SCR of NOx. The concept of Ag surface density allowed rationalizing some of the earlier results on C3H6-SCR of NOx.