Phase transitions and thermal history in CS2(HSO4)(H2PO4) were analyzed using powder X-ray diffraction method. A high-temperature phase (phase HT), which is a superprotonic phase, was retained on cooling even after 7 days in dry gas atmospheres at 298 K (room-temperature). On the other hand, phase HT did not transform to an original room-temperature phase (phase RT) but to another phase under room temperature and air atmosphere with 60% humidity. The new phase can be the same as the previously reported phase RT2 by means of H-1 NMR [S. Hayashi, M. Mizuno, Solid State Ionics 176 (2005) 745.]. It is revealed that water vapor in atmosphere affects the phase transition, resulting in these large thermal histories. (c) 2006 Elsevier B.V All rights reserved.