An investigation has been made of the fundamental factors which determine isotope separation factors on adsorbents, based on hydrogen and deuterium isotherms reported in Part I. Through the use of a semiempirical thermodynamic approach, the binary separation [111] is given. I t is suggested that the horizontal zero-point vibrations of the adsorbed species in some cases constitute an important contributing factor in addition to the vertical vibrations previously thought to be solely responsible for differences in isotope behavior.