Adsorption of H-2 on the zeolite Ca-A was investigated by a combination of variable temperature IR spectroscopy and calculations at the periodic DFT level. Calculations showed that adsorption complexes can be formed on two types of adsorption sites: Ca2+ in the center of six-member rings and Ca2+ in eight-member rings. Only the former adsorption complexes were experimentally observed in IR spectra; they are characterized by H-H stretching frequency of 4083 cm (1) and Delta H-0 = -12 kJ/mol in agreement with calculated results. The most likely explanation for not observing the adsorption complex on Ca2+ in 8-member rings is that the geometry of the complex does not render the H-H stretching mode IR active. (C) 2009 Published by Elsevier B. V.