Oxygen concentrations (C-b) and diffusion coefficients (D) in various proton exchange membranes were measured using chronoamperometry at microelectrodes. These measurements were made under conditions similar to those prevailing in proton-exchange-membrane fuel cells. Knowledge of the D and C-b parameters is essential for the determination of oxygen permeation in the catalytic layers of gas diffusion electrodes, which could be rate limiting in these low-temperature fuel cells. Furthermore, the D and C-b values also provide the permeation rates of oxygen through the bulk of the proton conducting membrane. It was found that the concentration of oxygen increased and the diffusion coefficient decreased with increasing equivalent weight of the membranes. These results were interpreted using a model based on the microstructure of the swollen membranes separated into two phases, one hydrophobic and the other hydrophilic, with distinctly different O-2-permeation properties. According to this model the relative amounts of the two phases in the membranes, caused by the different water contents, determine the O-2 solubility and diffusion coefficient in the different membrane materials.