The hydrogen storage properties of ZrMnFe0.45Co0.45V0.1 and ZrMn(0.9)Vo(0.1)Fe(0.5)Co(0.5), which are of C14 hexagonal structure, are studied in the ranges of 0.3 less than or equal to P (bar) less than or equal to 30 and 30 less than or equal to T (C-degrees) less than or equal to 100. The partial substitution of V at the Mn site or at the Fe/Co site in ZrMnFe0.5Co0.5 leads to the expansion of the lattice, the percentage of volume expansion being larger for ZrMnFe0.45Co0.45V0.1 than for ZrMn0.9V0.1Fe0.5Co0.5. The pressure-composition (P-C) isotherms of ZrMnFe0.45Co0.45V0.1 and ZrMn0.9V0.1Fe0.5Co0.5 show the existence of alpha, (alpha and beta) and beta regions. The hydrogen desorption isotherms of the alloys show that the hysteresis is small. The plateau pressures of ZrMnFe0.45Co0.45V0.1-H in the temperature ranges investigated, are lower than those in ZrMn0.9V0.1Fe0.5Co0.5-H, due to the larger size of the tetrahedral interstitial sites in the former alloy. The dependence of the relative partial molar enthalpy of dissolved hydrogen (DeltaH(H)) and the partial molar entropy of dissolved hydrogen (DeltaS(H)) in ZrMnFe0.45Co0.45V0.1-H and ZrMn0.9V0.1Fe0.5Co0.5-H on the hydrogen concentration show the presence of a, (alpha + beta) and beta phases. The chemical potential of dissolved hydrogen (Deltamu(H)) in ZrMnFe0.45Co0.45V0.1-H is larger than that in ZrMn0.9V0.1Fe0.5Co0.5-H due to the larger size of the tetrahedral interstitial sites in the former alloy. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.