In order to investigate the percolation behavior for ion transport, copolymers of styrene and methacrylic acid (MAA) were synthesized over a wide range of ion concentrations. The flux of K + through the ionomer membrane was almost independent of the feed K + concentration as far as the ratio of K + concentration to Na + concentration in the feed solution was maintained at constant. This was the characteristic for ionomer membrane. The permeability of K + increased discontinuously at about 14 mol% of MAA, which demonstrated the percolation behavior for K + transport. In the relation between the permeability and a difference of the volume fraction of MAA and water from the threshold volume fraction, a simple power law was confirmed. The critical exponent and the percolation threshold obtained for this system were 1.7 and 0.165, respectively, and these values were nearly in agreement with those expected for the randomly dispersed system. The copolymers of styrene and sodium methacrylate were well-known to form ion clusters. However, since our ionomer membrane was prepared without neutralization of the copolymer of styrene and MAA, it is considered that the carboxyl groups of the copolymer were undissociated at the time of the membrane preparation, and therefore the ion clustered structure was not formed. The result of DSC measurement suggested that the ionomer had no cluster structure.