The permeation of oxygen at atmospheric pressure through Nafion® 120 and NEOSEPTA® ACH‐45T ion‐exchange membranes was investigated by an electrochemical monitoring technique, which utilizes SPE composite electrodes prepared by an electroless plating method. The oxygen diffusion coefficients were almost the same for each material, but the oxygen solubility was much higher in Nafion than in NEOSEPTA. The oxygen solubility in NEOSEPTA could be explained in terms of dissolution in the aqueous component of the membrane, but the oxygen solubility in Nafion was too high for such an explanation, and was postulated to involve the role of the polytetrafluoroethylene backbone.