We prepared a new composite material composed of cesium hydrosulfate (CSHSO4) and phosphosilicate gel (P2O5-SiO2 gel); a high temperature form of the former exhibits exceptionally high proton conductivity at above 144 degrees C without humidification and the latter is one of the high proton conductors working under low humidity in a temperature range tip to around 150 degrees C. High electric conductivities of 10(-5) to 10(-2) Scm(-1) of the composite were observed under dry N-2 atmosphere in a wide temperature range from ambient to 160 degrees C and no steep change in conductivity at around 144 degrees C was observed. In X-ray diffraction patient of CsHSO4-(P2O5-SiO2 gel) composites, new diffraction peaks were observed and original peaks of CsHSO4 disappeared. A reaction of CsHSO4 and orthophosphoric acid (H3PO4), which is probably formed by the hydrolysis Of P2O5-SiO2 gel, would occur to produce a new crystalline phase with high proton conductivity at lower temperatures. (c) 2005 Elsevier B.V. All rights reserved.