The hydrogenation and dehydrogenation switching processes of Pd thin films were analyzed by measuring in situ in real time the variation in ellipsometric Psi and Delta during hydrogenation and dehydrogenation using spectroscopic ellipsometry. The hydrogenation proceeded from the film surface and a mixture layer of metal and hydride was formed at the surface. With time elapse, the thickness of the mixture layer increased rapidly. After the whole Pd metal layer transformed to the mixture layer, the concentration of hydride in the mixture layer increased. Finally, the concentration reached 1; the whole I'd in the mixture layer was hydrogenated, and the hydrogenation of Pd was complete. On the other hand, the dehydrogenation proceeded in a way completely opposite to the hydrogenation process: the concentration of hydride in the mixture layer decreased, Pd metal layer was formed at the interface between the film and the substrate, and the whole layer returned to Pd metal. The dehydrogenation proceeds much faster than hydrogenation. (C) 2009 Elsevier B.V. All rights reserved.