AC Impedance has been applied to solid oxide fuel cells at various operating conditions. Experiments have been performed on small (2 cm²) button cells and larger (61 cm²) planar cells. Button cells were fabricated with anode and cathode reference electrodes to facilitate separate testing of anode and cathode. Impedance spectroscopy has also been applied to a five cell planar stack and to single cells therein. Equivalent circuit models are derived with each circuit element representing different processes that limit cell output. Values for the elements are obtained by fitting the model to the experimental data. The degree to which electrode kinetics and mass transfer limit cell output may be evaluated from the fitted resistances, capacitances, and inductances of the circuit elements. The model (including the elements used, their arrangement, and the processes to which they are ascribed) is validated by changing cell temperature, reaction rate, and reactant concentration. The presented work is useful for performing stack maintenance and diagnostics and for designing process controls and power conditioning circuitry.