Gold–cobalt oxide (Au–Co ₃ O ₄ ) composite films have been prepared by the sputter-deposition of gold onto a glass plate substrate followed by pyrolysis of spin-coated cobalt 2-ethylhexanoate. The films comprise small Au particles and Co ₃ O ₄ nanocrystals, and exhibit different and independent optical responses to CO and H ₂ in air. Carbon monoxide caused only a decrease in absorbance, while H ₂ caused both a decrease and an increase in absorbance at different wavelengths. Among solid-state gas sensor materials, this Au–Co ₃ O ₄ composite film is probably the first example of an inorganic material which can be used for the recognition of CO and H ₂ molecules through optical absorbance changes.