The SECM has been used to investigate the influence of ionic strength on the ingress and egress of protons to and from a polyaniline (PANI) film. This was done in solutions containing 0.01 mol dm(-3) HCl, with concentrations of NaCl ranging from 0 to 0.5 mol dm(-3), by measuring the steady-state faradaic current for H+ reduction on a 18 mu m diameter Pt microdisc located within few micrometres of the PANI film while cycling the potential of the film through the different oxidation states. The microelectrode response was calibrated using microdiscs of various sizes in different solutions of HCl, with and without the presence of supporting electrolyte. This investigation shows the influence of ionic strength on the film mechanism and in particular on the relationship between the behaviour of H+ at the filmsolution interface and the behaviour of electrons at the filmelectrode interface.