A combination of variable-temperature infrared spectroscopy with volumetric gas adsorption measurements was used to study the thermodynamics of hydrogen adsorption, at a low temperature, on calcium-exchanged zeolites X and Y. Two adsorption regimes were considered: (i) localized adsorption of dihydrogen molecules on Ca(2+) cation sites, and (ii) delocalized hydrogen adsorption following saturation of the Ca(2+) adsorbing centres. For localized adsorption, the corresponding enthalpy change was found to be in the range of -12 to -15 kJ mol(-1), while the isosteric heat of delocalized adsorption was found to be in the range of 4.5-5.5 kJ mol(-1). These experimental results are discussed in the broader context of corresponding data for other alkaline zeolites, with a focus on correlation between adsorption enthalpy and entropy for the localized adsorption regime. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.