Proton and potassium ions countertransport across perfluorocarbon-type (Nafion) and hydrocarbon-type (K-101) cation-exchange membranes was studied, and the effect of proton transport on potassium ion transport was examined. It is considered that the fixed charge groups are distributed heterogeneously in the former and homogeneously in the latter. The experimental results were compared with calculated values using the Teorell-Meyer-Sievers＇s theory, which formulated transport phenomena based on the Donnan equilibrium and the Nernst-Planck flux equation. In the case of the K-101 membrane, the experimental results comparatively agreed with theoretical predictions. The deviation is attributed to the low estimation of the membrane effective charge density. On the other hand, the experimental results did not agree with theoretical predictions in the case of the Nafion membrane. It might be attributed not only to the low estimation of the membrane effective charge density but also to the high proton mobility caused by the charge transfer through structured water in the membrane pore.