Cs5H3(SO4)(4)(.)yH(2)O and (1-x)Cs5H3(SO4)(4)/xSiO(2) composite electrolytes (x=0.3 0.9) have been investigated by means of impedance, IR and Raman spectroscopy, differential scanning calorimetry and X-ray analysis. Cs5H3(SO4)(4)(.)yH(2)O has a phase transition to a highconductive disordered state (sigmasimilar to10(-2) S cm(-1)) at T-tr=418 K induced by changes in the structural-water content. This phase transition is shown to be reversible yet the converse transition from the high-temperature phase is slow. Although heterogeneous doping causes only a moderate increase in the low-temperature conductivity of the ionic salt, it stabilizes the high-conductive disordered state. The vibration spectroscopy data confirm the formation of the Cs5H3(SO4)(4) disordered state in composites. The composite conductivity does not depend on the composition up to x=0.7 and decreases at xgreater than or equal to0.8 due to the percolation effect. The structural dynamics of SO4 tetrahedra is shown to correlate with the proton conductivity. (C) 2004 Elsevier B.V. All rights reserved.