We analyzed oxygen permeability through the two different types of thin ionomer films, composed of Nafion and hydrocarbon (HC), on Pt surface. From the results of the number of permeated oxygen, density distributions of ionomer components and oxygen, and solubility distributions of oxygen, we have found that the number of permeated oxygen molecules decreases in both ionomers as water content increases. In the ionomer-gas interface and the bulk region of both ionomers, the oxygen solubility decreases, and solvent block the diffusion of oxygen with increasing water content. In the ionomer-Pt interface of the Nafionbased ionomer, the oxygen solubility also decreases, and the voids in the ionomer, which can be the pathways for oxygen, are occupied by solvent at higher water contents. On the other hand, in the interface of the HC-based ionomer, solvent molecules are piled up on the surface of high-density polymers, and block the permeation of oxygen.