A quantitative model has been developed for a recently introduced thin-layer method (Shi, C.; Anson, F. C. J. Phys. Chem. B 1998, 102, 9850; 1999, 103, 6283) for measuring the rate constants of electron transfer (ET) at the interface between two immiscible electrolyte solutions (ITIES). In this method a macroscopic graphite electrode is coated with a thin organic (nitrobenzene) film (phase 1) containing one redox-active species, which is immersed in a second immiscible (aqueous) phase (2) containing a different redox species, which is insoluble in phase 1. The model predicts the voltammetric response for ET processes at the ITIES, irrespective of the concentrations of reactants in the two phases. Results from the model show that an earlier approximate treatment of the method is rarely applicable for typical experimental parameters reported hitherto. The approximate method is only reliable when there is considerable excess of reactant in phase 2. Under these conditions, the current response for a bimolecular ET rate constant of 1 cm M-1 s(-1) can barely be distinguished from a diffusion-controlled process. The conclusions drawn from earlier experimental studies, suggesting that ET is independent of the potential drop across the ITIES, may be a consequence of this diffusional limitation. It is demonstrated that there are advantages to decreasing the concentration of reactant in phase 2, and in using a wide range of potential scan rates, to facilitate the determination of larger ET rate constants by this method.