Alternating current impedance measurements can be used as an in situ method for characterizing rate-limiting processes in fuel cell electrodes. Theoretical models have been derived and solved analytically. Theoretical analysis indicates that kinetic information can be extracted from the effect of temperature and gas composition on the size of the kinetic loop. The relative importance of kinetics and mass transfer can be inferred from AC impedance analysis. Several examples are given for the case of molten carbonate fuel cell (MCFC) electrodes. Such electrodes are not accessible to classical kinetic investigation techniques due to the high operating temperature and corrosive environment. Relevant kinetic information about MCFC electrodes has been obtained successfully using this in situ technique.