The diffusion of hydrogen in cubic (C15-type) Laves phase Ho(1-x)Mm(x)Co(2) (x=0, 0.2 and 0.4, Mm=mischmetal) alloys was measured in the alpha-phase (solid solution) region over the temperature range of 500-800 degrees C using Sieverts-type apparatus. The alloys and its hydrides have been characterized by powder X-ray diffraction (XRD) Rietveld method. The diffusion constants have been determined from the gas-solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy is obtained from the temperature dependence of diffusion using Arrhenius relation. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.