Addition effect of In and Pb on the NO-CO reaction over SiO2 supported Pd catalysts was studied, using a closed gas circulation system as well as in situ infrared, XRD and XPS spectroscopies. Formation of intermetallic compounds was observed in the cases of both Pd-In/SiO2 and Pd-Pb/SiO2 catalysts. These compounds caused a drastic enhancement of the reaction rate of N2O formation. Infrared analyses revealed the weakening of CO adsorption on Pd metal by the formation of intermetallic compounds, which is the main reason of the enhancement of reaction rate. Over Pd/SiO2, the reaction may proceed via redox mechanism through NO dissociation to form the oxidized surface, followed by its reduction with CO. By adding In or Pb, a new reaction pathway is opened at the lower temperature region via NO dimer-like intermediates. The role of additives is the stabilization of these unique intermediates on the slightly oxidized intermetallic compound surfaces. (c) 2007 Elsevier B.V. All rights reserved.