The role of Au nanoparticles inside Ti02-NiO sol-gel thin films for hydrogen sulfide optical detection has been assessed by combining optical spectroscopy, X-ray diffraction (XRD), reaction products analysis and X-ray photoemission spectroscopy (XPS) data. Sulfur oxides (i.e., SO2 and SO3, hereafter SOx) are clearly distinguished as reaction products and SOx, species are detected on the surface of the samples by XPS. After exposure to H2S, no metal sulfide species can be detected, neither by XRD nor XPS, so we propose that the reaction mechanism does not involve the formation of Ni, Ti or Au sulfides, but a direct catalytic oxidation of H2S to SOx. The role of Au is to provide an optical probe sensitive to the H2S presence, while the oxide matrix is responsible for the conversion of H2S to SOx.