Hydroxylation of the robust, weakly coordinating hexahalo- carborane anion system, CB₁₁H₆Br₆⁻¹, produces a new class of anion with mixed halo/hydroxyl substituents, HCB₁₁(OH)₅Br₆⁻¹ which can be used to isolate a number of hydronium cation salts including an ‘ice tautomer’ composed of hydronium cation, anion hydroxyl groups and coordinated water molecules.