In this work, the composition and morphology of WO3 films loaded with different levels of Ag, prepared by screen-printing onto Si substrates and annealed in air were investigated. The TEM micrography showed that the films are grain-like; the grain size increases with the increase of the Ag loading level. The Raman spectroscopy showed the formation of a AgWO3 bronze structure. XPS and ToF-SIMS results showed that while undergoing annealing, the Ag atoms migrate to the surface forming clusters. The molecular images obtained by ToF-SIMS showed that the NH3 binds preferentially at the surface of the Ag clusters. No preferential binding site was found for hydrocarbon contamination. (c) 2004 Elsevier B.V. All rights reserved.