Nd5.5W0.5Mo0.5O11.25-delta exhibits high total conductivity in reducing atmospheres and a significant H-2 permeation at high temperature. Nevertheless, the partial contribution from electrons, protons and oxide-ions in this material is still not reported. In this work, an exhaustive electrochemical study"has been performed for Nd5.5W0.5Mo0.5O11.25-delta . The total conductivity has been studied systematically in different environments, analyzing the influence of the temperature, pO(2) and pH(2)O. Transport numbers have been determined from the EMF measurement in concentration cells. H-2 permeation measurements for Nd5.5W0.5Mo0.5O11.25-delta have been performed from 725 to 625 degrees C and H/D isotopic studies were carried out at 700 degrees C. By coupling all these techniques, a complete overview of the transport mechanisms in the material has been achieved. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.