This paper develops and demonstrates a model representing radial defect transport through proton-conducting ceramic membranes, such as might be used in shell-and-tube type membrane reactors. The model uses a Nernst-Planck-Poisson (NPP) formulation and is designed to represent both steady-state and transient responses within mixed-conducting membranes with multiple charge-carrying defects. The partial differential equations, representing defect and charge conservation, are solved computationally using the method-of-lines in a differential-algebraic setting. Several example problems are solved and discussed, illustrating important aspects of the model. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.016304jes] All rights reserved.