Solid polymer electrolytes containing phosphotungstic acid (PWA) and/or silicotungstic acid (SiWA) in polyvinyl alcohol (PVA) were investigated for their proton conductivities. Enhanced conductivity was obtained when mixing PWA and SiWA at equal ratio. This polymer electrolyte was found viable for electrochemical capacitors. Thermal and structural analyses were conducted with DSC, XRD, and FTIR. The polymer electrolyte exhibited a different structure and different thermal properties from its respective components. The polymer electrolyte retained its original Keggin structure but contained crystallized protonated water in the form of H5O2 +. The protonated water may contribute to the proton conductivity of the polymer electrolyte.