The effect of thermal annealing as a pretreatment procedure for Nafion (R) membranes was investigated. Commercial membranes with different thicknesses and ages (all melt extruded films) were considered. After a short heat treatment at 165 degrees C, the proton conductivity, water permeability, equilibrium water sorption, and self-diffusion coefficient of water and protons increased in all films, with the greatest improvements occurring in thin films (< 50 mu m). Wide-angle X-ray measurements showed that the annealing procedure decreases the crystallinity of thick films and increases the crystallinity of thin films, in general. Small-angle X-ray measurements showed that annealing may alter the distribution of water within the films. Thermal annealing appears to 'normalize' the transport proper-ties of different thicknesses and batches of Nation (R), reducing the differences in values of measured transport properties between thin and thick membranes. (c) 2007 Elsevier B.V. All rights reserved.