Abstract: The kinetics of hydrogen adsorption on thin films in the two phase regions are studied. It can be classified as four steps: dissociation of hydrogen molecule and chemisorption on the gas/solid interface, surface penetration of hydrogen atoms, diffussion of hydrogen atoms through the hydride layer as well as hydride formation at the metal/hydride interface. It is shown by theoretical and experimental results that at low pressure, hydriding rate controlling step is mainly chemisorption, and at higher pressure( p H 2  p eq ) and approaching equilibrium of system, one of the other steps becomes rate determining step.