Thin films composed of Au nanoparticles dispersed inside a TiO₂-NiO mixed oxide matrix are prepared by the sol-gel method, resulting in nanostructured composites with a morphology and crystallinity that depend on synthesis parameters and thermal treatment. Their functional activity as hydrogen sulfide optical sensors is due to Au-localized surface plasmon resonance (SPR) which is reversible. The detection sensitivity is shown to be down to a few parts per million of H₂S, and almost no interference in response is observed during simultaneous exposure to CO or H₂, resulting in a highly sensitive and selective sensor for hydrogen sulfide detection. For mechanistic studies, experimental evidence using reaction product analysis and thin film surface characterization suggests a direct catalytic oxidation of H₂S over the Au-TiO₂-NiO nanocomposite film. ; peer reviewed: yes ; NRC Pub: yes