A method aimed at potentiometric estimation of the association of ions with ion-exchanger sites and charged ionophores in ion-selective electrode membranes is proposed. The method relies on the measurements of segmented sandwich membrane potentials. It is shown theoretically that the quantification of ion association requires use of weakly associated ionic additive whose concentration in the working segment of the sandwich must be varied. This is in contrast with well-established technique of ion to neutral ionophore complexation measurements. The advantages and limitations of the novel method are critically evaluated. Association of ions in plasticized poly(vinylchloride) membranes is studied experimentally. Experimental results are provided related to the association of K+, Na+, Cs+, and NH4+, and also Ca2+ with commonly used sites: tetra(p-Cl-phenyl)borate anion and calcium-selective lipophilic ion-exchanger bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate. (C) 2008 Elsevier Ltd. All rights reserved.