A Pt sheet or a free-standing polypyrrole (PPy) membrane has been studied in a Double Submerged Impinging Jet (DSIJ), in the presence of electrolytes containing redox couples. Impedance diagrams relative to PPy may be analysed as the result of two parallel paths involving (i) convective diffusion of the redox species in solution, electron transfer at the polymer/electrolyte interface, electron transport within the film and (ii) migration of non-electroactive ions in solution, ion transfer at the polymer/electrolyte interface, ion transport within the film. For a fast redox couple and a low ionic conductivity of the membrane, PPy explicitly shows an electrocatalytic effect with respect to Pt.