A mathematical model was developed for the impedance response associated with coupled homogeneous chemical and heterogeneous electrochemical reactions. The model includes a homogeneous reaction in the electrolyte in which species AB reacts reversibly to form A − and B + and B + reacts electrochemically on a rotating disk electrode to produce B. The resulting diffusion impedance has two asymmetric capacitive loops, one associated with convective diffusion and the other with the homogeneous reaction. For an infinitely fast homogeneous reaction, the system is shown to behave as though AB is the electroactive species. A modified Gerischer impedance was found to provide a good fit to the simulated data.