This paper presents studies concerning the optimization of PVC nitrate-selection membranes based on quaternary ammonium salts. Performances and selectivities of chemically modified field effect transistors based on the studied polymer membranes were tested. The influence of concentration and of the structure of the quaternary ammonium salts on the PVC membrane selectivity was investigated. The dependence of properties of the membrane on the polarity of the plasticizer used in membranes was also studied. The highest nitrate selectivity and almost theoretical response were obtained for o-NPOE/PVC membranes containing 1 percent of symmetrical tetradodecylammonium nitrate or tetradeclyammonium bromide.