Conductivity measurements of Nafion(R) 112, 115 and 117 membranes in the normal direction are reported in this paper. The measurements were made by means of impedance spectroscopy as a function of temperature. The conductivity was measured directly on hot-pressed carbon paper/membrane/carbon paper samples fully immersed in deionized water. The data show that Nafion(R) membranes are really isotropic and that tangential and normal direction conductivity measurements gave the same results when the same hydration level was utilized. (C) 2004 Elsevier B.V. All rights reserved.