Hydrogenation properties of MmNi(4.30-x)Co(x)Al(0.30)Mn(0.40) (x = 0, 0.75) were investigated by means of in situ X-ray diffraction (XRD). The XRD profiles were analyzed by the Rietveld method to evaluate the change of lattice parameters and lattice volume. Hydrogenation-dehydrogenation of these alloys proceeded through two phases between the solid solution phase and the hydride phase. Both the phases of MmNi(4.30)Al(0.30)Mn(0.40) had the hexagonal lattice with a space group of P6/mmm and those of MmNi(3.55)Co(0.75)Al(0.30)Mn(0.40) had the orthorhombic lattice with Cmmm after initial activation. In the two-phase co-existing region, lattice volumetric ratio of both the phases showed 19% for MmNi(4.30)Al(0.30)Mn(0.40), while that became 10% by partial substitution of Co for Ni with anisotropic lattice expansion. Anisotropic lattice strain related with high dislocation density was not introduced through formation of the hydride phase. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.