The surface composition of alloy particles in Pd-Ag/SiO2 hydrodechlorination (HDC) sol-gel catalysts has been determined by combining three experimental techniques, carbon monoxide chemisorption, X-ray diffraction, and transmission electron microscopy, Values obtained through this method show a very marked silver enrichment of the surface, as expected: as a consequence of the important gap between palladium and silver surface energies. These values are in complete agreement with surface composition curves calculated in other studies by means of Auger electron spectroscopy and infrared spectroscopy. The influence of sample pretreatment temperature as well as the influence of carbon monoxide on surface composition are discussed. (C) 2000 Academic Press.