Conductivity measurements of polycrystalline CsH2PO4 and CsH2PO4/silica composite were carried out in the temperature range from 150 to 250 degreesC under a humid condition. A steep change in the conductivity of polycrystalline CsH2PO4 due to a phase transition from a low conductive phase to a high conductive phase (superionic conduction phase) occurs reversibly at around 230 degreesC in the humid condition. It is found that the conductivity of CsH2PO4/silica composite is influenced by hydrophilicity and specific surface area of silica. The hydrophilic silica accelerates the conductivity of CsH2PO4/silica composite at the temperature nearly below the phase transition. In the composite system, impedance analysis showed two semicircles. It is considered that the high frequency semicircle is related to protonic conduction including bulk transfer and interface transfer between the ionic salt and silica, and the low frequency semicircle is possibly associated with grain boundaries. The accelerations of the conductivities can be induced by point defects generated in the ionic salt near the interface between ionic salt and silica. (C) 2003 Elsevier Science B.V. All rights reserved.