SiO2 sol–gel mesoporous thin films containing NiO nanoparticles have been synthesized and successively impregnated with Ag ions. A subsequent thermal treatment induces the precipitation of Ag nanoparticles and the formation of Ag/NiO interfaces. Morphological, structural and optical characterizations are used to analyze the matrix evolution with thermal annealing and to investigate the Ag+ impregnation procedure and the Ag nanocrystals precipitation. NiO nanoparticles form upon annealing at 800 ◦C while the SiO2 matrix is still highly porous due to the organic template removal. This porosity is useful for the impregnation process that is composed by a first pores functionalization with mercapto moieties and a second impregnation with Ag precursor solution followed by the thermal reduction of Ag ions. Optical gas sensing tests for CO and H2 detection are presented as one of the possible applications for these noble metal–metal oxides nanocomposites.