Metal borohydrides X(BH4)(n) (X = Li, Na, K, Ca) are attractive candidates for on-board hydrogen storage because they show high gravimetric capacity and high volumetric density. Hydrogen release from X(BH4)n (X = Li, K, Ca) are predicted to proceed via intermediate XnB12H12 (X = Li, K, Ca) phases. In the present study, we predict two new hydrogen storage reactions that are some of the most attractive among the presently known one. They combine high gravimetric densities with low enthalpies and they are thermodynamically reversible at low pressure due to low vibration entropies of the product phases containing the [B12H12](2) anion. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.