This study aims to better understand the Fe role in the hydrogen sorption kinetics of Mg-Fe composites. Mg-8 mol% Fe nanocomposites produced by high energy reactive milling (RM) for 10 h resulted in MgH2 mixed with free Fe and a low fraction of Mg2FeH6. Increasing milling time to 24 h allowed formation of a high fraction of Mg2FeH6 mixed with MgH2. The hydrogen absorption/desorption behavior of the nanocomposites reactive milled for 10 and 24 h was investigated by in-situ synchrotron X-ray diffraction, thermal analyses and kinetics measurements in Sieverts-type apparatus. It was found that both 10 and 24 h milled nanocomposites presents extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions, i.e., 300-350 degrees C under 10 bar H-2 for absorption and 0.13 bar H-2 for desorption. Nanocomposites with MgH2, low Fe fraction and no Mg2FeH6 are suggested to be the most appropriate solution for hydrogen storage under the mild conditions studied. (C) Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.