In Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolyser Cells (SOECs) the change of reactant gasses to product gasses is one of the factors that limit the performance of the cells. Therefore it is important to have a detailed understanding of the impedance associated with this change in the gas concentrations and how it relates to various operating conditions. In this paper we describe a SOEC tested as a steam electrolyser in a plug-flow setup. The measured gas concentration impedance increases during the test, even though the current, the inlet gas concentrations and the flow rates are kept constant. From the impedance arcs we conclude that the increase must be due to a build up of an uneven resistance distribution perpendicular to (and not parallel to) the gas flow direction in the Ni-electrode.