The small signal ac response of IrO2-based electrodes was investigated using a general impedance model for porous electrodes. This model consists in a distributed equivalent circuit that describes the transport and charge storage of electroactive species coupled with the reaction throughout the internal surface. The evolution of the impedance spectra with the variation in the dc-applied voltage was predicted, and the measurements obtained showed a very good agreement with the theory. The model allows the evolution of parameters related to transport, charge storage, and reaction, to be followed under widely varying conditions, from the capacitive domain to the region of heavy current flow by intense surface reaction. The active surface sites play a dominant role both in the capacitive charging process and in the transport component of the impedance.