The porous structure of Polikon K composite ion-exchange membranes based on polyacrylonitrile fibers has been studied by standard contact porosimetry. Differences in maximum porosity, internal specific surface area, macropore volume, and other structural characteristics attributed to the use of different molding pressures during the preparation of the membranes have been revealed. According to the analysis of the porometric curves measured in octane and water, it has been found that the volume of hydrophobic pores in the Polikon membranes is an order of magnitude lower than the total volume of hydrophilic pores. The ion selectivity of the membrane materials has been estimated from the porometric curves. A comparison of the ion transport numbers measured by the potentiometric method to the calculated values has shown that they are in good agreement.