Proton and lithium ion conducting polymer electrolytes, based on poly(vinyl alcohol)–H₃PO₄ and poly(ethylene oxide)–LiCF₃SO₃ plasticised with poly(ethylene glycol), have been used in the construction of electric double layer capacitors with both high density graphite sheet and activated carbon fabric electrodes. The polymer electrolytes have room temperature ionic conductivities (ca. 10⁻⁴–10⁻³ S cm⁻¹) that make them suitable for use in thin-film form in devices. The performance characteristics of the capacitors have been studied using impedance analysis, linear-sweep voltammetry and charge–discharge methods. The supercapacitors based on the activated carbon fabric have characteristically large values of overall capacitance of 360–470 mF cm⁻² [equivalent to single electrode capacitance 70–90 F (g carbon)⁻¹] with the proton conducting electrolyte. Systems based on lithium ion conducting polymer electrolytes, however, have a much lower capacitance of ca. 20 mF cm⁻² (equivalent to a single electrode capacitance of ca. 4 F g⁻¹).