Theoretical equations are developed for the second and higher harmonic responses of reversible and quasireversible redox reactions to an applied sinusoidal waveform as a function of frequency. Finite layer diffusion is taken into account in the theoretical development in order to describe the low-frequency response appropriately. The description is considerably simplified by the definition of a harmonic impedance function. Experimental results are presented for the redox systems ferrocene/ferricinium ion in tetraethylammonium perchlorate and ferrocyanide/ferricyanide in aqueous potassium chloride and are compared quantitatively with the theoretical results.