The diffusional flux of an alkali metal ion against its own concentration difference was observed in the presence of a pH difference across a perfluorinated carboxylate ionomer membrane. Differential behavior of Li+, Na+, and K+ ions in each counter transport against proton transport through the membrane was found. The internal solution contained 1 x 10(-1) mol dm(-3) alkali metal hydroxide and 1 x 10(-1) mol dm(-3) alkali metal chloride, and the external solution contained 2 x 10(-1) mol dm(-3) alkali metal chloride and HCl of various concentrations in the range of 1 x 10(-2) to 1 x 10(-1) mol dm(-3). The membrane potential rapidly changed in response to a pH jump in the external solution to reach an intermediate stage and then in a subsequent step it started to rise slowly to the final steady membrane potential. According to the assignments of the early fast step and the subsequent slow step in a time course to the changing process of the Donnan potential and the generation of intramembrane diffusion potential, respectively, the total membrane potential was divided into two constituents. The ion concentration at the membrane surface was obtained from the Donnan potential in each system. The diffusional flux was analyzed by using the ion concentration at the membrane surface and the intramembrane diffusion potential. By curve fitting of observed fluxes to theoretically calculated values, pK(a) for the acid dissociation constant of the COOH group in the perfluorinated ionomer membrane was obtained. The flux of the reverse permeation was ordered as Li+ > Na+ > K+. The enhancement in the system of Li+ ions has been interpreted in terms of the large value of the diffusion coefficient for protons rather than that for lithium ion itself. It has been seen that from the polarity of the diffusion potential generated within the membrane it accelerates the transport of alkali metal ion in each system. Because of the high ratio of the diffusion coefficient of protons to that of Li+ ions, the acceleration effect of the diffusion potential is pronounced for the Li+ ion permeation against the concentration. The pK(a) of the COOH group in the membrane was found to be 1.34,0.70, and 0.51 for H+-Li+, H+-Na+, and H+-K+ systems, respectively. This implies that pK(a) in the cluster-structured hydrophilic region for the Li+ ion system is apparently large due to the concentration effect of protons into the cluster region and that, for the K+ ion system, the effect of screening of the surface charge of the cluster by cations is large resulting in a shift of apparent pK(a) to the intrinsic value of the perfluorinated carboxy group.