Nafion alpha-relaxation is the subject of intense investigations as it regulates the performance of electric actuators and polymer electrolyte fuel cells. Dielectric spectroscopy and atomic force microscopy measurements of Nafion membranes allow identifying the conformation transition of the polymeric aggregates as the process underlying the alpha-transition. The dielectric permittivity curves of Nation membranes show that for relative humidity higher than similar to 60% and for temperatures higher than similar to 120 degrees C, the alpha-relaxation displaces to lower frequencies. Such unusual behavior is attributed to an elongation of Nation polymeric aggregates occurring at high temperatures (T > 120 degrees C) and is in perfect agreement with morphological changes inferred from atomic force microscopy analyses. (C) 2015 Elsevier B.V. All rights reserved.