AbstractThe desorption mechanism of as-milled 2NaBH4 + 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 °C at a heating rate of 3 °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.