ABSTRACTThe oscillatory change in the optical absorbance of NiO-TiO₂ film containing Au nanoparticles in the presence of H₂S gas are investigated. The oscillatory phenomena could be monitored by looking at the variation of the surface plasmon resonance peak of the Au nanoparticles embedded in the TiO₂-NiO matrix. Au nanoparticles act as optical probes in the detection of H₂S, while the oxide matrix is responsible for the catalytic oxidation of H₂S. To the best of our knowledge, it is the first time that oscillatory phenomena are monitored by optical spectroscopy.