Recognition of the reaction mechanism of hydrogen generation by borohydrides hydrolysis can provide effective help for optimizing the reaction condition and catalyst design. In the present work, the underlying mechanisms of BH4- spontaneous hydrolysis under water deficiency (2 water molecules) and water-rich (4 water molecules) condition were elaborated by the second-order Moller-Plesset perturbation theory (MP2) at MP2/6-311++G(3df, 3pd) level of theory. The present theoretical calculations predicted that dihydrogen bonding plays an extremely important role in the process of BH4- hydrolysis, and H-2 is formed by a hydridic B-H hydrogen and a protic O-H hydrogen. The hydrogen generation mechanism of BH4- hydrolysis in water-rich condition may experience BH4- -> BH3(OH)(-), BH3(OH)(-) -> BH2(OH)(2)(-), BH2(OH)(2)(-) -> BH(OH)(3)(-) and BH(OH)(3)(-) -> B(OH)(4)(-) four steps. BH4- hydrolysis mechanism in water-deficiency condition includes four steps in two possible reaction pathways as the water molecules participate with the reaction in different steps. All those reaction pathways with the same first step (BH4- -> BH3(OH)(-)) and its the Rate Determining Step (RDS) of the whole reaction. BH4- spontaneous hydrolysis at neutral or weak alkaline should be in the acid-catalyzed mechanism. (C) 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.