The crystal structure, the thermal analysis and the proton conduction in the superprotonic phase of Cs-2(HSO4)(H2PO4) have been investigated from 113 K to 423 K. A new structure model has been proposed in terms of the temperature change of XRD patterns. A phase transition was found in the superprotonic phase on the basis of the XRD and the thermal analysis in the low-temperature region. The phase transition shows a glass-transition-like behavior. The temperature dependence of proton conductivity has been discussed in relation to this transition. (C) 2015 Elsevier B.V. All rights reserved.