Internal steam reforming is an attractive option offering a significantly cost reduction and higher system efficiencies of a solid oxide fuel cell (SOFC) power plant. Furthermore, faster load response may characterise systems with internal reforming. However, complete internal reforming can lead to several problems, which can be avoided with partial pre-reforming of natural gas. For SOFC systems the ratio between internal and pre-reforming has to be optimised on the basis of experimental performance data. A detailed study concerning the pre-reforming in a reformer of considerable size (10 kW) is carried out. The influence of operating temperature and mass How variations on conversion of methane and ethane is analysed. During internal reforming of methane with substrate anodes, large temperature gradients were detected. Substrate samples with low catalytic activity show large response times with respect to parameter variations.