The apical-apical isomer of the polyhedral borane anion [a2(-)B(20)H(18)](4-) reacts with various proton sources in dichloromethane to produce the protonated species [a(2)-B20H19](3-) in high yield. The mixed tetraethylammonium-pyridinium salt of [a(2)-B20H19](3-) crystallizes in the monoclinic space group P2(1)/n with a = 11.223(3) Angstrom, b = 10.970(3) Angstrom, c = 15.328(4) Angstrom, beta = 106.06(1)degrees, V = 1819 Angstrom(3), and Z = 2. Data were collected on a Syntex diffractometer using Cu K alpha radiation, to a maximum 2 theta = 115 degrees, giving 2359 unique reflections, and the structure was solved by direct methods (SHELX86). The final discrepancy indices were R = 0.062 and R(w) = 0.086 for 1967 independent reflections with I > 3 sigma(I). The structure of the [a(2)-B20H19](3-) anion is similar to that of its apical-apical [a(2)-B20H18](4-) precursor with the addition of a hydrogen bridge connecting the two 10-boron atom cages. The boron-hydrogen-boron bond forms an angle of 91(3)degrees although the component B10H9 cages share a common axis through their B(1)-B(10) vertices.