Methanol decomposition has been studied by in situ Fourier transform infrared spectroscopy on a CdS/TiO2 bilayer deposited on glass substrates. First CdS thin films were deposited by chemical bath deposition method using the CdSO4 and thiourea as the precursors and ammonia as the complexing agent and have the optical band gap of 2.2-2.29 eV for the different CdSO4 solution concentrations. The structure of the CdS films was found to have mixed cubic and hexagonal phases, while the TiO2 films were polycrystalline anatase structure with an optical band-gap energy of 3.2 eV deposited at a total sputtering pressure of 0.1 Pa in an argon and oxygen gas mixture (Ar: O-2 = 80: 20) at 200 W sputtering power. The visible light active photocatalytic degradation efficiency of methanol over a TiO2/CdS double layer is four times higher than that of a TiO2 single layer. The decomposition of methanol pathway proceeds via formaldehyde as the intermediate product. (c) 2006 American Vacuum Society.