The acid H2B12(OH)(12) can be isolated as a crystalline solid by protonation of the hydroxylated borane anion, B-12(OH)(12)(2-). This acidic compound has low solubility in water, conducts protons in the solid state, and has thermal stability to a temperature of 400 degreesC. The conductivity mechanism is a Grotthuss mechanism with a low activation enthalpy (9-13 kcal/mol). This new acid represents an addition to the class of oxoacids, of which sulfuric and phosphoric acid are the most prominent examples.