We report on the steady state modeling of an experimental methanol fuel reformer for fuel cell applications. The fuel reformer consists of an AutoThermal Reformer (ATR) followed by an Oxygen Removal (OR) reactor, Steam Reformer (SR) and Water Gas Shift (WGS) reactor. The effluent from the WGS is fed to a series of three Preferential Oxidation (PROX) reactors that reduce the CO concentration to less than 40 ppm. A mathematical model of the reformer is developed and selected parameters of the model are fit to experimental data collected from a fuel reformer that was designed, built and operated by the Material and Chemical Research Laboratories (MCL) of the Industrial Technology Research Institute (ITRI) in Hsinchu, Taiwan. In order to develop a compact and high-performance fuel reformer system, the mathematical model is used to design a reformer that has the minimum possible combined volume of the steam reformer and water gas shift reactor. The result is that the volume of the optimized reactor units can be reduced by 17.2% without a significant change in the overall efficiency. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.