The heterogeneity of different cation-exchange membranes (Neosepta CMX, Selemion CMV, and HJC heterogeneous membrane) and their effects on transport properties were investigated using chronopotentiometry, membrane conductivity, and current–voltage curves. Modifying the classical Sand equation, a method has been developed to determine the fraction of the conducting region (ϵ) of the ion-exchange membrane. The ϵ values of the CMX, CMV, and HJC membranes were 0.93, 0.95, and 0.75, respectively. Considering the characteristics of each membrane—the CMX and CMV are reinforced homogeneous membranes, while the HJC is a heterogeneous membrane—the ϵ values determined in this study seem to be reasonable. The dependence of membrane conductivities and the limiting current densities on the fraction of conducting region of each membrane have also been studied.