A fuel processor, which incorporates a catalytic autothermal reformer (ATR), a sulfur trap, and a water-gas-shift (WGS) bed engineered to. thermally integrate the ATR and WGS processes for improved thermal efficiency, was developed and tested with, natural gas as a hydrogen generator for fuel cell applications. In this study, the fuel processor was successfully run over 2300 h of continuous operation at 3.1-16.0 kWth input energy capacities. Test results obtained from characterizing the ATR performance show that the reformer achieved over 40% H-2 (dry basis) in the ATR reformate and 96-99.9% methane conversion over the entire test duration. The methane reforming efficiency reached over 90% at 16.0 kWth input energy level (or equivalent to 29 L/min fuel processing rate), but decreased with decreasing fuel processing rate. The oxygen-to-fuel molar ratio and the fuel processing rate were found to be the key operating parameters in determining the ATR bed temperature, the methane conversion, and the methane reforming efficiency. © 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.