The catalytic performance of bimetallic rhodium-silver clusters for the selective catalytic reduction of NO by propylene has been examined over 1%(Rh-Ag)/Al2O3 catalysts of variable Ag content. It was found that substitution of small amounts of Rh by Ag results in a significant increase of the catalytic activity, which goes through a maximum for catalysts with metal compositions of ca. 95% Rh-5%Ag. This behaviour is tentatively attributed to the formation of rhodium-rich phase alloys, which may stabilize Rh in its reduced state at reaction conditions.