New equations for oxygen transport, which is driven by the chemical potential gradient through an ionomer thin film on a Pt surface, were developed. The chemical potential was described by two terms which indicated the effect of the oxygen concentration and that of the interaction between oxygen molecules and the ionomer film on the oxygen flux. The oxygen transport resistance of the ionomer/Pt interface was the largest, which is suggested to be attributed to the structural properties; the density of the ionomer/Pt interface is very high compared with the rest of regions, and the oxygen concentration is lower since the oxygen molecules are distributed locally in the ionomer/Pt interface while the oxygen molecules are distributed widely in the rest of regions. The oxygen transport resistance of the ionomer/Pt interface was underestimated while that of the ionomer/gas interface was overestimated if one considers only the concentration gradient as the driving force. Therefore, the oxygen-film interaction was found to have a significant effect on the oxygen transport resistances of the both interfaces.