To better understand the sizing and performance issues associated with water and energy balances of a fuel processor for a PEM fuel cell, a process flow system has been programmed into the ASPEN (R) simulator for a reference system supplying 600 W electrical power. The fuel processor consists of an autothermal reformer (ATR), single-stage water gas shift (WGS), dual-stage PROX reactors and an anode gas burner (AGB) operating on sulfur-free JP8. The reactor components were modeled using RGIBBS for the ATR and actual experimental kinetics for the WGS and PROX reactors. Simulations investigating thermal management, ATR and WGS performance and system efficiencies were done. Using realistic temperature approaches for heat exchangers it is likely that integrated systems will have to discard heat to the surroundings. Also, depending on the operating conditions, water will either be in surplus or deficit. At 1 atm pressure there is no condition that will enable complete water capture. The minimum pressure where water independence can be achieved is at 1.23 atm with a fuel cell utilization of 60% and a S/C = 1.5 and O/C = 1.0. The complete fuel processor is expected to be 0.46 L and 2.02 kg. This paper will expand on these findings with suggestions for optimal operating efficiencies and sizing for JP8 fuel processors. (c) 2007 Elsevier B.V. All rights reserved.