New composite free standing films were fabricated by solution casting poly(diethyl, dipropyl)phosphazene, phosphoric acid and a naphthalenic sulfonated copolyimide. The proton conductivity data in dry environment obtained for these films allow to conclude that the presence of strong sulfonic acid proton donors and of phosphazene N proton acceptor sites is not sufficient to allow good conductivity in dry environment, and that the content of phosphoric acid appears the most critical factor for the proton mobility. (c) 2007 Elsevier B.V. All rights reserved.