Pore size of Neosepta CMX (Tokuyama Soda, Japan), Selemion CMV (Asahi Glass, Japan) and heterogeneous HJC cation-exchange membrane (Hanguk Jungsoo, Korea) was characterized by chronopotentiometry in NH4Cl and in other amine chloride solutions - simple homologous amine chloride, R-NH3Cl, where R = CH3, CH2CH3, (CH2)(2)CH3, (CH2)(3)CH3, (CH2)(4)CH3, and (CH2)(5)CH3. The chronopotentiometric curves obtained in the CMX and CMV membranes showed a steep inflection while diffusive in the heterogeneous HJC membrane. In all membranes, the inflections of chronopotentiometric curves became diffusive with the enlargement in molecular size of transporting ions. Assuming that the ion-exchange membrane consisted of conducting and non-conducting phases, Sand's equation was modified in this study. Using the modified equation, the fraction of inert material contained in the membrane and the pore size were determined by comparing the theoretical and experimental transition times. The pore size of the membranes, determined by the chronopotentiometry method, correlated with membrane conductivity. It was considered that the diffusive inflection in the chronopotentiometric curve of the heterogeneous membrane was caused by the delay in the formation of the concentration gradient. This delay, in turn, was due to a high fraction of non-conducting region.