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Relative contributions of double‐layer and faradaic impedances have been studied for two typical cases involving important electrochemical processes. The faradaic impedance has been interpreted quantitatively in terms of models which describe either the mass transport or the reaction mechanism at the electrode. The first case is the electrochemical reduction of the I3− ion at a rotating platinum disk in the presence of a supporting electrolyte (KI 0.1N). The mass transport impedance has been computed by means of a Laplacian transformation of the impulsional response of the system. It is shown that this impedance accurately follows an expression based on Nernst's hypothesis. The diffusion layer thickness, determined by means of impedance measurements, is in agreement with that deduced from hydrodynamic data. The second case is the anodic dissolution of iron in aqueous sulfuric acid at different pH's. The steady‐state current‐potential curves and the complex impedance in the whole frequency range, between 30 kHz and 10−5 kHz, have been interpreted in terms of a dissolution mechanism in two steps involving an adsorbed normalFeOH intermediate.