The hydrogen storage properties and the thermodynamics of dissolved hydrogen in the C14 hexagonal Laves phase Zr0.9Ho0.1MnFe0.5Co0.5 and Zr0.9Ho0.1MnFe0.5Ni0.5 alloys are studied. The partial replacement of Ho in Zr site in ZrMnFe0.5Co0.5 and ZrMnFe0.5Ni0.5 alloys leads to the expansion of their lattices. The pressure-composition (p-c) isotherms of Zr0.9Ho0.1MnFe0.5Co0.5 and Zr0.9Ho0.1MnFe0.5Ni0.5 in the pressure and temperature ranges 0.6 less than or equal to P(bar) less than or equal to 20 and 30 less than or equal to T(C-degrees) less than or equal to 100 show sloppy plateau (alpha + beta) region. The average relative partial molar enthalpy of dissolved hydrogen at the (alpha + beta) region in Zr0.9Ho0.1MnFe0.5CO0.5-H and Zr0.9Ho0.1MnFe0.5Ni0.5-H are -15.5 kJ (mol H)(-1) and -14.5 kJ (mol H)(-1). respectively. Similarly the average relative partial molar entropy of dissolved hydrogen at the two-phase region in Zr0.9Ho0.1MnFe0.5Co0.5-H and Zr0.9Ho0.1MnFe0.5Ni0.5-H are -51 J K-1 (mol H)(-1) and -50 J K-1 (mol H)(-1), respectively. The presence of alpha, (alpha + beta)-and beta-phases as seen in the p-c isotherms, is seen from the powder diffractograms of the alloy hydrides. The kinetics of hydrogen absorption in these alloys are quite fast which is between 15 and 20 min at room temperature. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.