Proton-conducting composites of heteropolyacid hydrates (phospbotungstic acid, PTA and phosphomolybdic acid, PMA) with dispersoids such as insulating Al2O3, Al-2(SO4)(3).16H(2)O and (NH4)(10)W12O41.2H(2)O are prepared for use as possible solid-state electrolytes in batteries. Bulk electrical conductivity as a function of composition is reported. Rechargeable solid-state proton batteries are fabricated and characterized. A cell with the configuration Zn + ZnSO4.7H(2)O + MHx\PMA + APT\PbO2 + V2O5 + C + E gives an open circuit voltage of 1.5 V and can run for >850 h at a current drain of 2.4 muA cm(-2). The cell can be recharged without much loss up to 18-20 cycles.