High efficiency reforming is a key parameter of high temperature stationary fuel cell systems. In this study, a planar heat exchanger steam reformer (PHESR) was integrated with a catalytic combustor in order that the unused energy of the anode off-gas is delivered for heating and reforming. The PHESR was designed to use the anode off-gas of the externally reformed SOFC system because it has an efficiency problem. In the PHESR reactor, the heat is transferred from the catalytic burner to the reformer that has the smallest gradient of temperature difference between the two reactors. The thermal behaviors of the exothermic and endothermic reactions between the reactors were investigated experimentally. The parameters of the investigation were fuel utilization, inlet temperature, and air excess ratio. The comparison parameter was the volume fraction of hydrogen at the exit of the reforming side. The temperature gradients of two reactors were measured in the longitudinal direction. As expected, the temperature differences between the two reactors were crucial factors that required optimization. Furthermore, the geometric aspects between the finned and un-finned reactors were also investigated. The results demonstrate that the volume fraction of hydrogen at the exit is closely coupled with the geometric constraints and operating parameters. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.