Mg2CoH5 was synthesized by reactive mechanical milling (RMM) under hydrogen atmosphere (0.5 MPa) from 2MgH(2)-Co and 3MgH(2)-Co mixtures, with a yield >80%. The microstructure, structure and thermal behavior of the phases formed during the processing were investigated by transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Kinetic properties of the reaction with hydrogen of the 2MgH(2)-Co and 3MgH(2)-Co mixtures after RMM were evaluated using modified Sieverts-type equipment. The 3MgH(2)-Co mixture showed better properties for storage applications, with its highest rate of hydrogen absorption and desorption at 300 degrees C, its storage capacity of about 3.7 wt% in less than 100 s, and good stability after cycling. Although the starting material presents Mg2CoH5 as majority phase, the cycling leads to disproportion between Mg and Co. We obtained a mixture of Mg2CoH5, Mg6CO2H11 and MgH2 hydrides, as well as other phases such as Co and/or Mg, depending on experimental conditions. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.