Solvent polymeric membranes based on quaternary ammonium salt tridodecylmethylammonium chloride (TDDMAC1) with variable concentration of lipophilic inert electrolyte (ETH 500), plasticized with solvents of different nature were investigated by potentiometry and electrochemical impedance spectroscopy. DOS-plasticized membranes exhibited increased selectivity to divalent anions in agreement with the phase-boundary potential model. Conductivity mechanism in such membranes was proposed on the basis of the data on conductivity, activation energy of conductivity and pre-exponential factor and assuming the formation of electrolyte-containing water droplets in the membrane, facilitated by ETH 500.