Catalytic mechanism at thin film-modified electrodes is studied both theoretically and experimentally under conditions of square-wave voltammetry (SWV). The electrode system considered consists of a solid electrode covered with an electrochemically inactive thin film (e.g. a layer of a water-immiscible organic solvent) containing a neutral redox probe (R1). The modified electrode is immersed into an electrolyte solution (typically aqueous electrolyte solution) that contains a second redox probe (R2), present in a large excess compared to R1. The product of the assumed one-electron electrode reaction O1+, confined within the film, is transformed back to the initial reactant R1 by means of a catalytic redox reaction with the R2. If the permeation of the R2 in the thin film is significant, the catalytic reaction undergoes as a homogeneous process confined within the boundaries of the thin film. Otherwise, the catalytic reaction undergoes as a heterogeneous process involving electron transfer across the interface between the thin film and the electrolyte solution.