Highly dispersed Au, Ag and Au-Ag alloy nanoparticles were homogeneously embedded in a chemically inert matrix made of silicalite-1 multi-hollow single crystals. The particles were fully accessible to small reactant molecules such as CO. The Au nanoparticles were as active as the Ag-Au material for CO oxidation at room temperature. Operando diffuse reflectance FT-IR spectroscopy revealed the presence of both metals at the surface of the particles, silver replacing a large fraction of the Au atoms otherwise located at low coordination sites. This study shows that CO adsorption followed by IR is a useful technique to estimate the surface composition of nanoalloy particles buried in a zeolite matrix.