Magnesium silicide (Mg(2)Si) has attracted interest as a hydrogen storage material due to favorable thermodynamics (Delta H(desorption) = 36 kJ/mol H(2)) for room temperature operation. To date, direct hydriding of Mg(2)Si under hydrogen gas to form MgH(2) and Si has only been attempted at low pressure and has been hindered by poor kinetics of absorption. In this paper we study the dehydrogenation reaction with in-situ neutron powder diffraction and present results of our attempts to hydrogenate Mg(2)Si under both hydrogen and deuterium gas up to temperatures of 350 degrees C and pressures of 1850 bar. Even under these extreme absorption conditions Mg(2)Si does not absorb any measureable quantity of hydrogen or deuterium. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.