[en] The influence of Mm (Mm=mischmetal) substitution and hydrogen absorption on the magnetic properties of Ho_1-xMm_xCo₂ (x=0, 0.1, 0.2, 0.3 and 0.4) alloys have been determined through the temperature dependence of ac susceptibility and thermopower measurements. The changes in magnetic-ordering temperature of Ho_1-xMm_xCo₂ alloys have been explained based on the dilution of the magnetic ions and weakening of 4f-3d exchange interactions. The gradual disappearance of the magnetic transition temperature upon increasing hydrogen concentration (y) has been interpreted by the lattice expansion and charge transfer between absorbed hydrogen and 3d-band of Ho_1-xMm_xCo₂