The desorption mechanism of as-milled 2NaBH(4) + MgH2 was investigated by volumetric analysis, X-ray diffraction and electron microscopy. Hydrogen desorption was carried out in 0.1 bar hydrogen pressure from room temperature up to 450 degrees C at a heating rate of 3 degrees C min(-1). Complete dehydrogenation was achieved in two steps releasing 7.84 wt.% hydrogen. Desorption reaction in this system is kinetically restricted and limited by the growth of MgB2 at the Mg/Na2B12H12 interface where the intermediate product phases form a barrier to diffusion. During desorption, MgB2 particles are observed to grow as plates around NaH particles. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.