The primary objective of this paper is to develop the theoretical underpinnings for a model that predicts the performance of protonic-ceramic fuel cells (PCFC). Such fuel cells have been demonstrated to perform well with hydrogen and hydrocarbon fuels in the intermediate temperature range of 400 <= T <= 700 degrees C. Because the electrolyte materials are typically doped perovskite ceramics (e.g., BaZr0.9Y0.1O3-delta, BZY10) that are mixed ionic-electronic conductors (MIEC), the model formulation is considerably more complex than is the case for solid-oxide fuel cells (SOFC) that use electrolyte materials such as yttria-stabilized zirconia (YSZ) that are purely oxygen-ion conductors. The model considers transport and chemistry within porous composite electrode structures that are comprised of an electronically conducting phase and an MIEC phase. The defect transport within the MIEC phases is represented using a Nernst-Planck formulation. Using a button cell configuration with nominal material properties and cell geometry, the paper exercises a computational model to demonstrate the model and explore a range of operating conditions. (c) 2017 The Electrochemical Society. All rights reserved.