The combination of small Au particles with NiO, CuO and Co3O4 films led to improved or new optical gas sensitivity via different mechanisms. The deposition of small Au particles on NiO film enhanced the detection of CO in air through a decreasing change of absorbance in the whole visible-near IR region. This can be explained by the change in positive hole density change of NiO, which was enhanced by an increase in catalytic formation and decomposition of surface carbonates with Au. In the case of GO-insensitive CuO film, gold deposition made it sensitive only at plasmon absorption region. This might be because the absorbance by small Au particles in plasmon band was influenced by the change in the dielectric constant of copper oxide which surrounded small Au particles. A Au-Co3O4 composite film showed a novel function to recognize CO and H-2, probably for the first time as inorganic materials for optochemical sensors. This owes to the fact that CO and H-2 bring about the specific absorbance change of the film at different wavelengths. Carbon monoxide decreased the absorbance of Co3O4 moiety, while H-2 not only decreased the absorbance of Co3O4 moiety but also increased plasmon absorbance of Au.