Thin-layer electrochemical reactors are analytical reactors where mass transfers are generally assumed not to affect the overall reaction rate. The validity of this hypothesis was tested for systems involving a second-order or an enzymatic homogeneous reaction that consumed the product generated by the electrochemical heterogeneous reaction. The experiments were performed with the oxidation of hexacyanoferrate II into hexacyanoferrate III, coupled with the consumption of hexacyanoferrate III by the reduced form of nicotinamide adenine dinucleotide, catalyzed or not by a diaphorase. The tools commonly used for gas- liquid interfaces were adapted to the description of mass transfer in a closed thin film. The theoretical model agreed well with the experimental data and served as the basis for testing the thin-layer hypothesis. The validity of the hypothesis was plotted as a function of the Hatta and Damkohler numbers. The influence of concentration and diffusion parameters are discussed with focus on the validity criterion.