The hydrogenation behaviour of RNi5/CO (R = Ce, La) bi-layer thin films has been investigated. To prevent the poisoning of surface of thin films due to the gases like O-2, CO2, etc. the films were well coated by a thin over layer of cobalt because of its resistance to oxidation. Thin film samples were characterized by means of XRD. In order to investigate the effect of hydrogen on the surface structure of the film, the optical micrograph and transmission spectra of as deposited and hydrogenated thin film samples have been carried out. It can be concluded from the transmittance spectra that the hydrogenation of the film causing an increment in the transmission. Thin metallic films switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous hydrogen. It has also been confirmed from the change in optical micrographs of the films. It has been found that the resistance of film increases with the absorption of hydrogen. The resistance of the same decreases with desorption of hydrogen. The effect of temperature and hydrogen exposing time on the resistance of the film has also been studied. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.