The influence of the background concentration of ZnCl2 on the selectivity of Zn(NCS)42–−SE based on QAS to SCN− is studied. The introduction of ZnCl2 into a solution fixes zinc released from the membrane in the form of thiocyanate complex. A high selectivity of the Zn(NCS)4 2–−SE to SCN− ions is due to the formation of better extractable complexes between zinc and SCN− compared to acidocomplexes of competitive anions. An introduction of 5.0 M ZnCl2 into a solution increases selectivity to SCN− ions by 5 orders of magnitude in the presence of interfering ClO4 − ions and more than 4.5 orders of magnitude in the presenceof interfering NO3 − ions.