The effect of phenols containing different electron-withdrawing substituents on the potentiometric responses of several liquid PVC membranes containing the complex trioctylmethylammonium-nitrate, TOMA(+)NO(3)(-), is analyzed. The results make it possible to separate these phenols into two groups; those phenols containing electron-releasing groups, which produce almost Nernstian slopes, and those containing electron-withdrawing substituents, which generate sub-Nernstian slopes. The highly negative standard free energy of transfer of the aryl phenolic group from water to a cationic polar head suggests that these phenols are mainly located in the membrane phase associated with TOMA(+) via a cation-pi interaction. It seems that the strength of this interaction, and hence of the nitrate dissociation, is affected by the presence of phenols in an extension which correlates well with the kind of phenol present in these membranes and, consequently, with the type of their potentiometric responses. (C) 2003 Elsevier Inc. All rights reserved.