The influence of MgCo on the reaction paths during hydriding and dehydriding processes of Mg-Co mixtures was studied using a combined HP-DSC and XRD approach. Mg-Co mixtures with different compositions were mechanically milled under argon to prepare Mg-Co nanocomposites and then submitted to thermal treatment at 300 degrees C for 5 days to induce MgCo formation. The local Mg-Co composition in the milled and milled-heated samples determines the nature of the phases obtained after hydriding/dehydriding cycling. The formation of Mg6Co2H11, Mg2CoH5 and MgH2 hydrides occurs after the first hydriding stage of the 2Mg-Co and Mg-Co milled mixtures due to kinetic restrictions. On the contrary, Mg-Co milled-heated mixture exhibits the selective formation of Mg2CoH5 during first hydriding via two-step reaction. In the first one, MgCo disproportion to MgH2 and Co takes place simultaneously with Mg hydriding (<200 degrees C). The second step involves MgCo hydriding to Mg2CoH5 through MgH2 as intermediate phase (>200 degrees C). Dehydriding reaction is enhanced by dispersion of Co into Mg-matrix, which reduces more than 100 degrees C the hydrogen desorption temperature when compared with the Mg-Co milled sample without previous heating. (C) 2009 Elsevier B. V. All rights reserved.