A direct relationship is derived between the charge-transfer resistance and the resistive terms ascribable to diffusion for a faradaic reaction influenced by transport of the reacting species to the electrode. The charge-transfer resistance is shown to approach a finite value for potentials at which the current is limited by mass transfer and, conversely, the diffusion impedance approaches a finite value when the current is controlled by kinetics. Supporting experimental results are presented for both an irreversible (oxygen reduction reaction) and a quasi-reversible (ferrocyanide oxidation) electrochemical systems investigated with a rotating- disk electrode.