The stoichiometric mixture of 2MgH(2) + Co was ball milled under a hydrogen atmosphere to synthesize nanocrystalline metal hydride Mg2CoH5. Upon milling, the mixture was analyzed by X-ray powder diffraction (XRD) and thermal methods employing the techniques of differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermal analysis (DTA). Hydrogen absorption and desorption measured by pressure-composition-temperature (P-C-T) curves indicated that the capacity loss was small after 20 consecutive cycling tests. The enthalpies associated with hydride formation and decomposition were measured to be -69.5 and -83.2 kJ mol(-1) H-2, respectively. At the temperatures of this study (553 to 653 K), hysteresis decreases with increasing temperature.