In this work, a new substitution mechanism was proposed to describe the dehydrogenation of the hand-mixed LiBH4-MgD2 mixture. Above the melting point of LiBH4, the free D atoms originated from the decomposition of MgD2 would replace the H atoms of liquid LiBH4, leading to the formation of B D bond, and major release of H-2 and HD other than D-2 gas. The hydrogen substitution mechanism would play the role above the melting point of LiBH4. Similar hydrogen substitution effect was found in the LiBH4-VD2 mixture, not in the NaBH4-MgD2 mixture. Due to the substitution mechanism, the dehydrogenation kinetics of MgD2 in the LiBH4-MgD2 mixture is enhanced, which is contrary to the retarding effect due to hydrogen exchange mechanism previously proposed in the ball-milled LiBH4 MgD2 composite. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.