Hydrogen absorption isotherms for the CaCu5 hexagonal structured alloys MmNi3.8Al0.4Fe0.4Co 0.4, MmNi3.5Al0.5Fe0.5Co 0.5, MmNi3.4Mn0.4Al0.4Fe 0.4Co0.4, MmNi3.368Mn0.333V 0.333Al0.3Fe0.333Co0.333 and MmNi3Mn0.333V0.333Al0.333Fe 0.333Co0.333Cu0.333 with Fe and Co in the mole ratio 1:1 have been obtained in the temperature and pressure ranges 30 ≤ T/°C ≤ 100 and 0.1 ≤ P/bar ≤ 30 using a high pressure unit based on the pressure reduction method. The powder X-ray diffractograms of the unannealed alloys show the formation of single phase. The lattice constants and the unit cell volume of these alloys increase with substitution of Mn, V, Cu, Fe, Al and Co at the Ni site in MmNi5. The hydrogen absorption isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied. The hydrogen absorption studies show no marked change in the plateau slope due to the presence of Fe and Co in 1:1 mole ratio. The maximum hydrogen intake capacity (r = nH/n f.u.) is around 4.75 in MmNi3.8Al0.4Fe 0.4Co0.4 at 30 °C and at 30 bar. The dependence of the thermodynamics of dissolved hydrogen in these alloy hydrides on the hydrogen concentration shows the different phase regions α, α+β and β seen in the hydrogen absorption isotherms. At any particular temperature investigated, the chemical potential of dissolved hydrogen in these alloy hydrides decreases with the substitution of Mn, V, Cu, Fe, Al and Co at the Ni site, which has been correlated with the increase in the volume of the unit cell of the alloys. The desorption isotherms of MmNi 3.8Al0.4Fe0.4Co0.4 at 30 and 50 °C show that the hysteresis is very small with free energy loss per cycle about 0.4 kJ/mol H at 30 °C. The kinetics of hydrogen absorption at 30, 50, 75 and 100 °C have been studied for MmNi3.8Al 0.4Fe0.4Co0.4. The different phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms. The average activation energies Ea in the α, (α+β) and β phases are found to be 0.38 eV, 0.32 eV and 0.054 eV, respectively. The diffusion coefficient at 303 K in MmNi 3.8Al0.4Fe0.4Co0.4-H is about 2.55 × 10-10 cm2 s-1. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo any structural transformation upon hydrogenation. © 2003 Elsevier B.V. All rights reserved.