A technique for separating cationic and anionic constituents of the ionic conductivity of amorphous solid polymer electrolytes in the region of low salt concentrations is proposed. The technique employs model equations for separately describing transport of cations and anions that differently interact with the polymer matrix. It is shown that the temperature dependences of the anionic conductivity obey the Vogel-Tamman-Fulcher equation and those of the cationic conductivity, the Miyamoto-Shibayama equation. By separating the overall ionic conductivity into constituents, cationic transport numbers are evaluated in broad ranges of salt concentrations and temperatures in the system comprising a copolymer of acrylonitrile and butadiene (40:60) and lithium hexafluoroarsenate. At nearly ambient temperatures the anionic constituent is predominant. Two different cation transport mechanisms are established at elevated temperatures.