The stabilization of superprotonic phase in neat CsH2PO4 and CsH2PO4/SiO2 composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH2PO4 at around T-c = 230 degrees C was confirmed under humidified and sealed conditions. In CsH2PO4/SiO2 composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (Cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH2PO4 and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices. (c) 2008 Elsevier Ltd. All rights reserved.