Abstract Polymer films were deposited electrochemically on a platinum electrode surface, either by constant current density electrolysis for polypyrrole or by potential scan for polyphenol and polyaniline. The permeability of these modified electrodes to oxygen, Fe(CN) 6 3− and NADH was measured by constant potential electrolysis. Although it was possible to control to some extent the polypyrrole permeability versus oxygen and Fe(CN) 6 3− by choosing the electropolymerization conditions, the film became almost totally impermeable to larger species such as NADH as soon as its thickness exceeded a few monolayers. A new protocol was proposed, which consisted in adding inert molecules such as polyethylene glycol (PEG) to the electropolymerization solution. These molecules induced change in the polymeric matrix morphology. 70% of the NADH flux obtained at a clean electrode was then recovered with a 5 nm-thick polypyrrole modified electrode. The experimental conditions, i.e., the molar mass and the concentration of PEG, were optimized; the maximum polypyrrole permeability of2.0 × 10 −3cm s −1 was recorded with PEG (1000 g mol −1). These conditions also allowed an enzyme such as glucose oxidase to be satisfactorily confined at the electrode surface. Similar results were obtained with polyphenol and polyaniline or when PEG was replaced by dextran molecules.