The appearance of an extra are in impedance spectra obtained on high performance solid oxide fuel cell (SOFC) anodes is recognized when experiments are conducted in a test setup where the working and reference electrodes are placed in separate atmospheres. A simple continuously stirred tank reactor (CSTR) model is used to illustrate how anodes measured with the reference electrode in an atmosphere separate from the working electrode are subject to an impedance contribution from gas conversion. The gas conversion impedance is split into a resistive and a capacitive part, and the dependences of these parameters on gas composition, temperature, gas flow rate, and rig geometry are quantified. The fuel gas flow rate per unit of anode area is decisive for the resistivity, whereas the capacitance is proportional to the CSTR volume of gas over the anode. The model predictions are compared to actual measurements on Ni/yttria stabilized zirconia cermet anodes for SOFC. The contribution of the gas conversion overpotential to de current-voltage characteristics is deduced for H-2/H2O and shown to have a slope of RT/2F in a Tafel plot.