Molecular dynamics simulations were performed to analyze the water content dependence of the structural properties of ionomer and the oxygen permeation properties in ionomer on a Pt surface. The ionomer/gas interface, bulk region, and ionomer/Pt interface were defined based on the density distribution of ionomer. Oxygen pathways are clearly observed in the ionomer/Pt interface for quite a long period. It was found that the oxygen permeability decreases with increasing water content, which is the opposite of what takes place in the Nafion membranes. Then the oxygen diffusivity, solubility, and permeability in each region were analyzed. It was found that the oxygen permeability in the ionomer/Pt interface was dominant in the oxygen permeability in the ionomer. Furthermore, a decrease in the oxygen solubility has a larger effect on the water content dependence of oxygen permeability. Considering the structural properties of the ionomer, the oxygen solubility in the ionomer/Pt interface decreased because the voids were blocked at higher water content. (C) 2017 The Electrochemical Society. All rights reserved.