Thin-film composites comprised of NiO and NiO/Au nanoparticles in a porous SiO2 matrix have been prepared using the sol–gel technique. When at elevated temperatures (200 ◦C < T < 350 ◦C) and exposed to carbon monoxide, the films undergo reversible changes in optical transmittance at wavelengths in the visible–near IR region. For NiO composite films heated at 330 ◦C and exposed to 1% CO in air, there is an increase in transmittance which approaches 2–4% over most of the visible range. For NiO/Au composite films the transmittance increase exhibits a wavelength dependence, with a maximum change which is close to 6% at λ ≈ 630 nm and which is close to zero in the Au plasmon resonance range (λ ≈ 550 nm).