X-ray powder diffraction studies were performed on Cs-3(HSO4)(2)(H2PO4) and Cs-4(HSO4)(3)(H2PO4) in the range 340-440 K. For the both compounds a first order phase transition from the monoclinic C2/c phase to the mixture of cubic Pm-3m and tetragonal I4i/amd superprotonic phases takes place at 408-414 and 412-414 K respectively; the ratio between cubic and tetragonal depends on material and temperature. The tetragonal phase transforms to the cubic modification on heating and this transition is reversible.