The permeation behavior of tetraalkylammonium chlorides and C.I.Basic Red 18 was investigated. The permeability coefficient, P against the concentration of the upstream salt solution, Cu was quite different between tetraalkylammonium chlorides and C. I. Basic Red 18. Tetraalkylammonium chlorides gave an usual pattern of P on C. The small value of P due to Donnan exclusion in the low concentration range increases with the increase of Cu. On the contrary, C. I. Basic Red 18 gave a decrease of P with dye concentration. The result of tetraalkylammonium chlorides can be interpreted by means of TMS theory. The effective charged group concentration in the membranes was found to depend on the ionic species. The greater the Stokes radius the larger the effective charge density of the membranes. The result of C. I. Basic Red 18 can be analyzed in terms of the dual mode interaction between polymer and dye. The difference of the permeation behavior between tetraalkylammo-nium chlorides and C. I. Basic Red 18 is attributed to the higher affinity of the larger dye ion.