An extended three-wire model of the conductivity of ion-exchange membranes is proposed. The model includes a set of three-wire model equations supplemented with coupling equations between three-wire model parameters and the parameters of a microheterogeneous model. To calculate the complete set of parameters for evaluating the structural inhomogeneity of a membrane and calculating its effective electrotransport properties, it is necessary and sufficient to get only the concentration dependence of the specific conductivity of the membrane, which simplifies the characterization procedure. The model was verified based on independent experimental data and the calculations of transport-structural parameters with the use of the microheterogeneous model. It was found that the proposed model approach makes it possible to obtain a reliable combination of structural parameters to characterize the volume fractions of conductive phases and their mutual arrangement in the ion exchange material peculiar to the membrane test sample.