Thin electrolyte films are frequently used in electrochemical systems. We have developed a powerful tool to reveal the current distribution in such films using segmented Pt array electrodes. In this study, this electrode system was applied to a polymeric film of an anion-exchange ionomer. We found that current distributions for oxygen reduction varied widely with the thickness of the ionomer films. A combination of experimental and simulation studies revealed that the transport of ions and oxygen molecules governed the currents in ionomer films.