A systematic study of the structural phase transition of NH3BH3 and of its fully deuterated analogue was performed combining DSC and anelastic spectroscopy measurements. The transition is accompanied by a latent heat, and therefore is of the 1st order. On the deuterated sample the enthalpy variation is reduced of more than 20%, from 1.29 to 1.01 kJ/mol and the transition is shifted by similar to 1.5 K toward higher temperatures. Both NH3BH3 and ND3BD3 display a temperature hysteresis between cooling and heating, thus denoting that the phase transition is of first-order. In addition, this hysteresis is extremely small (similar to 0.5 K) indicating that the coexistence region between the two phases is very narrow. During isothermal ageing, the transformation of the low-temperature orthorhombic phase into the high-temperature tetragonal one occurs with a time constant of similar to 16 min in NH3BH3 and similar to 64 min in ND3BD3, evidencing a drastic slowing down of kinetics in the deuterated compound. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.