HYDROGEN isotope diffusion in metals and polymers has generally been treated in the literature as a single body problem with the motion of the hydrogen regarded as the important parameter. On this basis the equation for interstitial diffusion of hydrogen atoms in metals or molecular diffusion in polymers follows an equation of the form where D is the diffusion coefficient, v a frequency which corresponds to the vibration of the atoms or molecule in the solvent lattice, d the distance traversed in an elementary displacement and E the diffusional activation energy1. For the single body approximation, v is inversely proportional to the square root of the isotopic mass since hydrogen is much lighter than the molecules or atoms with which it interacts, so that