Investigations of solid solutions of CsHSO4 and CsH2PO4 have led to the discovery of a new compound with a sulfate to phosphate ratio of 2:1. Single-crystal X-ray data showed the as-synthesized phase to be monoclinic and not isostructural with any known compounds. Ac impedance measurements revealed that, upon heating, the compound undergoes a transformation into a phase of high conductivity at similar to 119 degrees C. The activation energy dropped from 0.90(2) eV to 0.45(5) eV, while the conductivity jumped from 2.5 X 10(-5) Ohm(-1) cm(-1) to 6.3 X 10(-3) Ohm(-1) cm(-1). Powder diffraction data suggest that the high-temperature phase is body-centered cubic. Although unlikely, the possibility of a Cs+ contribution to the conductivity at elevated temperatures cannot be precluded. Upon cooling, the compound remained in the high conductivity phase until 60-80 degrees C. At this point it transformed into a third, new phase and the conductivity dropped almost three orders of magnitude, but remained higher than in the as-synthesized compound. Given the similarities between this new compound and CsHSO4, it is likely that high-temperature proton conduction in Cs-3(HSO4)(2)(H2PO4) is facilitated by sulfate and/or phosphate ion reorientations.