A new method for determining selectivity coefficients of carrier based ion-selective electrodes toward highly discriminated ions is proposed. For each separately measured ion solution, the interfacial layer of the membrane contacting the sample must contain only the ion to be measured for the experimental determination of Nikolskii coefficients that reflect the true ion-exchange selectivity of the membrane. For this reason, it is recommended here that the electrode membrane is conditioned in a solution oi a discriminated ion that, preferably, is equal to the cation oi the tetraphenylborate salt incorporated in the membrane. The selectivity of valinomycin based plasticized poly(vinyl chloride) membrane electrodes has been characterized by using this new method and logarithmic Nikolskii coefficients for K+ over Na+, Mg2+, and Ca2+ have been determined as -4.5, -7.5, and -6.9, respectively. All measured ions showed near-Nernstian electrode slopes.