A theoretical study of a planar and self-breathing fuel cell is presented. This work contains the development of a mathematical model for planar self-breathing fuel cells, the validation of the model, and a study of the behavior of this type of fuel cell. The mathematical model presented is two-dimensional and nonisothermal. The validation of the model is performed by comparison of the measured overall performance of a planar self-breathing fuel cell to the predictions of the model. For this type of cell, the maximum power density is in the range between 0.5 and 0.4 V, so the model is applied to study the behavior of the reference cell at a cell voltage of 0.4 V. The results of this study show the gas distribution, the potential distribution, and the temperature distribution are influenced strongly by the geometric design of the cathode end plate. The charge generation rate in the active area of the cathode and anode is affected by the ribs of the cathode end plate. A strong nonuniformity of the current distribution in the cathode is found. © 2004 The Electrochemical Society. All rights reserved.