Impedance spectroscopy of PtBaCe0.8Gd0.2O3Pt cells indicates that both mass and charge-transfer influence the electrode kinetics of the interfacial reactions. Charge-transfer appears to dominate the electrode kinetics when the partial pressure of oxygen is sufficiently high, whereas mass transfer seems to be rate limiting at low oxygen partial pressures. Under the influence of an applied de bias, concentration polarization at the cathode is more severe than that at the anode. In a H-2-containing atmosphere, the electrode kinetics seem to, be limited primarily by charge-transfer. Further, water vapor has a significant effect on the electrode kinetics. In an O-2-containing atmosphere, the introduction of 3 volume percent (v/o) H2O dramatically increases the interfacial resistances. In contrast, in a H-2-containing atmosphere the presence of 3 v/o H2O significantly reduces the interfacial resistances. Overall, the interfacial resistances are the factors limiting the performance of intermediate-temperature solid oxide fuel cells based on BaCe0.8Gd0.2O3 electrolytes. Minimizing polarization resistance arising from both charge and mass transfer is the key to achieving high-performance solid oxide fuel cells.