This paper presents a lumped element model of a natural gas steam reformer being part of a 5 kW(el.) residential fuel cell power plant. It describes the thermal behaviour over the whole operating range including plant startup. For the reforming reactions chemical equilibrium is assumed. The derivation of the model starting with basic thermal and chemical laws is shown. Therefore model parameters have a physical meaning and were calculated with the assumption of lumped system properties. To achieve a better agreement with experiments the parameters were furthermore identified from measured data. Experimental data of a demonstration plant were used to verify the model. (c) 2005 Elsevier B.V. All rights reserved.