The relation between cell voltage (V-Cell), applied chemical potential difference (Delta mu(O-2)) and cell current (I-t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I-i) and overpotential (eta) The theoretical relations are used to analyze experiments on fuel cells with Ce0.8Sm0.2O1.9 and C0.8Gd0.2O1.9 electrolytes with La0.8Sr0.16CoO3 or Pt as the cathode and Ni/Ce0.9Ca0.1O1.9-x or Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.