Hydrogen absorption pressure-composition isotherms have been investigated for the ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys having the C14 type hexagonal structure in the temperature and pressure ranges 30≤T≤100°C and 0.1≤P≤40 bar using a pressure reduction method. Powder X-ray diffractograms show that the lattice constants and the unit cell volume of ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys decrease with increasing Ni content. The P-C isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied and it was found that the plateau pressure and plateau slope at any given temperature increase with increasing Ni content except for ZrMnFe0.5Ni0.5, where the plateau slope is found to be small due to the atomic ordering in the alloy due to presence of Fe and Ni in a 1:1 molar ratio. The desorption isotherms of ZrMnFe0.5Ni0.5 at 30 and 50°C show that the hysteresis is very small. The dependence of the thermodynamics of dissolved hydrogen in ZrMnFe1 - xNix-H (x=0.2, 0.4, 0.5 and 0.6) systems in the temperature range 30-50°C on the hydrogen concentration have shown the existence of α, α+β and β phase regions as seen in the isotherms. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo a structural transformation upon hydrogenation. In ZrMnFe0.5Ni0.5, the kinetics of hydrogen absorption during activation and after several cycles at 30 °C have been studied and compared. © 2002 Elsevier Science B.V. All rights reserved.