Various mono- and polyheme c-type cytochromes are able to spontaneously incorporate within thin films made from the ionomer poly(ester-sulfonic acid) Eastman AQ-29D acting as cation-exchange films cast on carbon electrodes. AQ-films as evidenced from scanning electron microscopy are uniform. The electron transfer within this cation-exchange film is strongly affected by the film thickness. The performances of the AQ-films to incorporate cytochromes by varying pH are examined on the basis of electrostatic interactions which principally govern the incorporation process. From film-transfer experiments, a model consisting of a two-phase ionomer coating has been developed for cytochrome c, An indirect approach based on the study of the incorporation of nonelectroactive proteins and peptides is reported. The respective abilities of AQ-29D and Nafion films to incorporate/not to incorporate c-type cytochromes and other ＇low molar mass＇ species are explained by the formation of hydrated gellike AQ-films.