The permeabilities and diffusion behaviors of aniline, p-chloroaniline, and p-nitroaniline were investigated in polysulfonated ICE-450 ion-exchange membranes as a function of pH at 25 degrees C. The permeabilities for these compounds increased with decreasing of receiver side pH. The modes of membrane/solution distribution and diffusion of amines, as a controlling factor to the membrane permselectivity in terms of basicity value, molecular size, and electrophilic groups of amines, were correlated consistently with the flux data as well as with the estimated membrane permselectivity parameters. The transport system depends on the basicity values of amines, and the pH gradients were the main driving force. It is shown that the diffusion parameters measured on single component experiments provide a good prediction of behavior.