Mixtures of CsHSO4 and CsH2PO4 were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs-3(HSO4)(2)(H2PO4) and Cs-5(HSO4)(3)(H2PO4)(2) were formed during milling. The mechanochemically treated composite consisting of Cs-3(HSO4)(2)(H2PO4) and Cs-5(HSO4)(3)(H2PO4)(2) showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs-2(HSO4)(H2PO4) was generated from the composite at around 100 degrees C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 degrees C on heating. The value becomes 2 x 10(-3) S cm(-1) at around 180 degrees C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs-2 (HSO4)(H2PO4). (c) 2006 Elsevier B.V. All rights reserved.