Au-Ag nanoporous systems were prepared by a recently developed method. Contrary to the electrochemical dealloying, the methodology applied in this work proved that it allowed to prepare bimetallic AuAg NP systems with controlled chemical compositions. Compared with monometallic Au or Ag catalysts, the alloy catalysts exhibited high activity toward CO and/or H-2 oxidation and exceptionally high selectivity at low temperature for CO oxidation in the presence of H-2 showing a synergistic effect between Au and Ag. Among the different characterizations, Low-Energy Ion-Scattering Experiments provided composition data about the topmost atomic layer, i.e., where catalytic reactions occur and allowed us to establish a clear correlation between top layer surface concentration and reactivity. A pronounced dependence of the reaction rate on the silver concentration has been observed only for H-2 oxidation. This strong dependence has been correlated with the number of surface sites constituted of adjacent Ag atoms required for dissociative oxygen adsorption. The high selectivity of the Au-Ag alloys with respect to pure gold has been ascribed to a large segregation of silver, especially on low coordination sites, inhibiting the H-2 adsorption. (C) 2013 Elsevier Inc. All rights reserved.