Effects of pretreatment on the surface of alumina-supported Pd-Ag catalysts with oxygen or oxygen-containing compounds (NO, N2O, CO and CO2) have been studied using an improved X-ray photoelectron spectroscopy (XPS). Surface analysis was performed either before or after the selective hydrogenation of acetylene. Analysis of the surface after reduction shows evidence of a Pd-Ag alloy. The binding energy of the Pd 3d is not affected by pretreatment, whereas a significant shift of the Ag 3d is revealed after NO and N2O pretreatment. The surface after reaction shows no state change of either Pd or Ag compared to those measured prior to reaction, which is in agreement with the reactivity test; therefore surface modification occurs after pretreatment and is retained even after 8 h on stream. No carbonaceous deposits are formed after 8 h on stream. Ethylene gain enhancement by NO and N2O pretreatment is a result of strong adsorption on the surface which may block the sites responsible for ethylene hydrogenation without facilitating carbonaceous deposits for hydrogen spillover. On the other hand, pretreatment with O-2, CO or CO2 increases the Pd active sites, which increases C2H2 hydrogenation activity. (C) 2004 Elsevier B.V. All rights reserved.