Impedance measurements were used to evaluate the differential capacity and the charge transfer resistance of the o-nitrophenyl octyl ether (o-NPOE) plasticized PVC membrane I water interface over the range of potential differences from -280 to 180 mV and the range of temperatures from 290 to 305 K. The equilibrium potential difference at the membrane I water interface was controlled by the partition of tetramethyl-, tetraethyl-, tetrapropyl- or tetrabutylammonium ion. Soaking of the PVC membrane in water for up to 8 h does not have an effect on the impedance parameters, which points to an absence of water sorption into the membrane in the presence of hydrophobic electrolytes. The differential capacity of the membrane I water interface is comparable with that of the o-NPOE I water interface, and its dependence on temperature throws some light on the role of the surface dipole. Lower apparent rate constants of ion transfer across the membrane \ water interface (k(a)(o) approximate to 10(-2) cm s(-1)) can be a linked to the activation barrier associated with a modification of surface layer at the o-NPOE \ water interface by PVC molecules.