investigations into the CsHSO4-CsH2PO4 system have yielded a new solid acid, Cs-2(HSO4)(H2PO4), with a superprotonic phase transition that occurs over the temperature range 61-105 degreesC. In the room temperature structure, the SO4 and PO4 groups me randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs-2(HSO4)(H2PO4) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, DeltaH = 44+/-2 J/g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with a(o) = 4.926(5) Angstrom, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO4 groups (X = P or S) at the center. The conductivity in the high temperature phase at 110 degreesC is 3 x 10(-3) Ohm (-1) cm(-1), and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO4 group reorientations in the cubic phase of Cs-2(HSO4)(H2PO4), as is known to occur in the high temperature, tetragonal phase of CsHSO4.