Ternary Mg2THy, as well as quaternary Mg2T0.5T'(0).H-5(y), Mg-based hydrides with T. T' = Fe, Co, Ni were synthesized by reactive mechanical alloying in hydrogen atmosphere at 0.3 MPa. The reaction progress was followed from the H-2 absorption during milling until a steady state was reached, and different kinetic behaviors were found in each case. These regimes could be related to the presence or absence of hydrogen-free Mg2T intermetallic precursors in the respective phase diagrams. The steady state samples were characterized by X-ray diffraction and differential scanning calorimetry, and those systems including Fe were studied, in addition, with Mossbauer spectroscopy (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.