The first mechanistic investigation of a topochemical dihydrogen to covalent bonding conversion is presented. Solid-state decomposition of the LiBH4. TEA (TEA = triethanolamine) dihydrogen-bonded complex into a covalent material was studied using B-11 solid-state MAS NMR, FT-IR, XRD, and optical microscopy. The majority of this solid-state reaction occurs by nucleation and two-dimensional growth of the covalent product nuclei. Variable-temperature kinetics and H/D exchange experiments established that proton transfer between the OH groups of the TEA and the BH4- anions, at the reactant/product interface, is the rate-limiting step, with an associated activation barrier of 21.0 +/- 2.4 kcal/mol. The activation parameters Delta H-double dagger and Delta S-double dagger for the same process were calculated to be 20.1 +/- 2.4 kcal/mol and -16.8 +/- 6.2 eu, respectively, comparable with the analogous values found for the aqueous hydrolysis of BH4- in neutral water, suggesting similar mechanisms fur the solid and solution decompositions.