The Frumkin isotherm for the electrosorption reaction is S-shaped for the interaction parameter values lower than a critical level, leading to a hysteresis loop during steady-state studies. It is shown that isotherm branch switching phenomena can occur during the study of the electrosorption reaction by electrochemical impedance spectroscopy. Two- and three-part discontinuous impedance diagrams can be observed when the sinusoidal potential modulation spans the discontinuity potentials, depending on the frequency sweep direction.