Arginine is an amino acid which bears a positive electrical charge at neutral pH. We studied the equilibrium properties of a cation exchange membrane widely used in electrodialysis, the CMV Selemion membrane in contact with aqueous solutions containing both sodium chloride and arginine chlorhydrate. The water content of the membrane, the amount of each ionic species present in the membrane and the individual mobilities of sodium and arginine ions in the membrane were determined as a function of the arginine concentration (10(-3) to 10(-1) M) in the external aqueous solution. The last parameter was obtained by combining the electric resistance and self-diffusion flux measurements. The mobility of the sodium ion was lowered by the presence of arginine inside the membrane material. This result was analysed taking into consideration the variation of the water content in the membrane containing the amino acid.