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. The coupling of homogeneous chemical reactions and heterogeneous electrochemical reactions has drawn substantial interest over the past 80 years. Koutecky and Levich 1-3 developed a steady-state model for a homogeneous reaction coupled with an electrochemical reaction on a rotating disk electrode. The homogenous reaction was assumed to follow linear kinetics, all diffusion coefficients were assumed to be equal, and the Schmidt number was assumed infinite. Koutecky and Levich defined a characteristic dimension for the homogeneous reaction, termed the thickness of the kinetic layer and represented bywhere α = k f +k b , k f and k b are respectively the forward and backward rate constants of the homogeneous reaction, and D is the diffusion coefficient. Bossche et al. 4 describe finite-difference calculations under assumption of a steady state for an electrochemical system controlled by diffusion, migration, convection, and nonlinear homogeneous reaction kinetics. Their convection term used a three-term expansion appropriate for positions close to the electrode surface.5 Deslouis et al. 6 used a submerged impinging jet cell to measure interfacial pH during the reduction of dissolved oxygen in the presence of carbonate. Their analysis considered the homogeneous reaction involving water and hydroxide, bicarbonate, and carbonate ions. Remita et al. 7 have shown that, for a deaerated aqueous electrolyte containing dissolved carbon dioxide, hydrogen evolution is enhanced by the homogeneous dissociation of CO 2 . Tran et al. 8 demonstrated that homogeneous dissociation of acetic acid enhances cathodic reduction of hydronium ions. Smith 9,10 used AC Polarography to study different linear firstorder homogeneous reaction mechanisms, including preceding, following, and catalytic chemical reactions coupled with electrochemical reactions. Jurczakowski and Polczynski 11 developed an AC model with coupled homogenous and heterogeneous reactions accounting for cases where diffusion coefficients are not considered to be equal. The above mechanisms assumed simplified homogeneous reactions, with a maximum of two species considered.Using chronopotentiometry, Delahay and Berzins 12 showed that cadmium cyanide complexes undergo a dissociation before electrore- * Electrochemical Society Student Member. * * Electrochemical Society Fellow. * * * ...