The leucine permeability coefficients, P, through a cation exchange membrane were measured as a function of HCl concentration, C-HCl, from 0 to 10(-1) mol/L for three different interfacial conditions and were compared with the results for glycine. Leucine transport phenomena remarkably depended on the interfacial condition between the membrane and the external solution. The lowest P (2.33 x 10(-8) cm(2)/s) was obtained at C-HCl = 0 for both sides of the membrane, and generally P increases with an increase in C-HCl. In case of the glycine, the lowest P (1.85 x 10(-7) cm(2)/s) had been obtained if C-HCl On the glycine source side was 10(-1) mol/L and that on the opposite side was 0, The highest P (4.34 x 10(-7) cm(2)/s) was obtained if C-HCl On the leucine source side was 0 and that on the opposite side was 10(-1) mol/L as well as for glycine (4.16 x 10(-6) cm(2)/s). The P values of glycine had been explained by the transport mechanism based on the interfacial chemical reaction (protonation, deprotonation, or ion-exchange) and the ionic transport across a charged membrane, The P values of leucine were also explained qualitatively by the same mechanism as glycine; however, they are remakably affected by the physicochemical properties of the membrane, such as hydrophobicity and molecular size.