This paper focuses on modeling membrane-polarization processes within mixed-conducting electrolyte membranes. Because a complete set of thermodynamic and transport properties is available, the electrolyte material used for the study is a yttrium-doped barium zirconate (BaZr0.9 Y-0.1 O3-delta, BZY10). Unlike Polymer Electrolyte Fuel Cells (PEMFC) and Solid-Oxide Fuel Cells (SOFC), the open-circuit voltage cannot be evaluated using gas phase compositions alone. Using a Nernst-Planck-Poisson (NPP) model, an important aspect of the present paper is to develop the theory needed to evaluate open-circuit potential. Focusing on the mixed-conducting membrane alone, the present model neglects all activation and concentration overpotentials. The model is exercised in both fuel-cell and electrolyzer modes, revealing significantly different polarization behaviors. The model based results show how the membrane polarization depends upon operating conditions, including temperature and gas-phase compositions. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.