A novel proton conductor has been developed using ternary polymer blend. Polymer films cast-coated from solutions containing poly(acrylic acid), poly(vinyl butyral) and vinylidene-fluoride-based copolymer demonstrated higher ac conductivity than that of a Nafion((R)) recast film. The relationships between (i) polymer composition and membranous character and (ii) polymer composition and ac conductivity have been investigated. As a result, it has been proven that the ternary polymer film has good thermal stability and high proton conductivity. Based on that, an optimised polymer composition has been determined. (C) 2004 Elsevier Ltd. All rights reserved.