Steady state transport through mixed proton, oxygen ion and electron/hole conductors is. examined in fuel cell, driven fuel cell, and electrolyzer modes. Oxygen ion and proton current densities and the corresponding effective permeation fluxes of H-2 and O-2 obey Onsager equations. Chemical potentials of electrically neutral species, mu(H2), and mu(O2), in the electrolyte are determined in terms of transport parameters and operating conditions. In fuel cell mode mu(H2), and mu(O2), in the electrolyte are bounded by electrode values. However, in electrolyzer and driven fuel cell modes, mu(H2), and mu(O2) in the electrolyte need not be bounded by electrode values. Under some conditions mu(H2), and mu(O2), in the electrolyte exceed electrolyte thermodynamic stability leading to device failure. The analysis shows that from the standpoint of stability, electrolytes exhibiting mixed ionic-electronic conducting properties with large interfacial electron transfer resistances are preferred over pledominantly ionic conducting materials. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.