Thermal behavior and a mechanism of the proton conduction were investigated for a composite with an in-situ formed Cs2H5(SO4)(2)(PO4) and a phophosilicate gel. The raw material compositions investigated were 0.4CsHSO(4) - xP(2)O(5)-(0.6 - x)SiO2 with x = 0.20, 0.25, and 0.30, The melting point of Cs2H5(SO4)(2)(PO4) decreased with increasing x, implying that Cs2H5(SO4)(2)(PO4) was microscopically present in the composite. The composite with x= 0.30 showed a high conductivity of 9 x 10(-3) S cm(-1) even after keeping at 120 degrees C for 1300 min under dry argon atmosphere. The Arrhenius plot of the conductivity for the composite was divided to three temperature regions, each of which has the distinct activation energy. Although the composite showed about 1.5 orders of magnitude higher conductivity than one of the constituent phase of Cs2H5(SO4)(2)(PO4), the activation energy of the conductivity for the composite, similar to 40 kJ mol(-1), was equivalent to that for Cs2H5(SO4)(2)(PO4) in the middle-temperature region, revealing that the proton conducted through Cs2H5(SO4)(2)(PO4) phase, not phosphosilicate gel. The structural disordering of Cs2H5(SO4)(2)(PO4) at the interface with the phosphosilicate gel would have raised the conductivity. The difference of the activation energy of the conductivity among three temperature regions was attributed to phase transitions of Cs2H5(SO4)(2)(PO4). The newly formed P- O - P bonds were shown to be present in the composites from the P-31 MAS NMR spectra; however, how these bonds affected the proton conductivity remain unclear. A dispersion in addition to the dc conductive plateau appeared in the conductivity spectra for the composite, revealing that the Coulomb interactions act between the mobile protons. A scaling analysis of the conductivity spectra showed the temperature-independent concentration of the mobile proton. (C) 2012 Elsevier B.V. All rights reserved.