Investigations of the system CsHSO4-CsH2PO4 led to discovery of the new compound, beta-Cs-3(HSO4)(2)(H-x(P,S)O-4), with a superprotonic phase transition at 125 degrees C. The room temperature structure of beta-Cs-3(HSO4)(2)(H-x(P,S)O-4) is related to that of alpha-Cs-3(HSO4)(2)(H2PO4) and that of CsHSO4-II. It is unique, however, in that P and S occupy the same crystallographic position in the structure, and there is accordingly a variable H content and a partially occupied hydrogen bond. This leads to room temperature conductivity that is significantly higher than either of the two related compounds. The conductivity in the high temperature phase is comparable to that of CsHSO4-I. Furthermore, the transition exhibits a relatively large heat of 30 J g(-1) and some hysteresis. These an explained by probable disordering between S and P sites upon transition to the superprotonic state.