Five CHB11X6Y5- carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH3 were generated by electrospray ionization, and their reactivity with a series of Bronsted acids and electron transfer reagents were examined in the gas phase. The undecachlorocarborane acid, H(CHB11Cl11), was found to be far more acidic than the former record holder, (1-C4F9SO2)(2)NH (i.e., Delta H degrees(acid) = 241 +/-29 vs 291.1 +/- 2.2 kcal mol(-1)) and bridges the gas-phase acidity and basicity scales for the first time. Its conjugate base, CHB11Cl11-, was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 +/- 0.02 eV) but the value for the (1-C4F9SO2)(2)N- anion is even larger (6.5 +/- 0.1 eV). Consequently, it is the weak H-(CHB11Cl11) BDE (70.0 kcal mol(-1), G3(MP2)) compared to the strong BDE of (1-C4F9SO2)(2)N-H (127.4 +/- 3.2 kcal mol(-1)) that accounts for the greater acidity of carborane acids.