Streaming potential variation with pressure measured through poly(ethylene terephthalate) track-etched membranes of different pore sizes led to the determination of an apparent interfacial potential zetaa in the presence of 10-2 M KCl. The variation of zetaa with the pore radius r0 is interpreted by (i) the electric double layer overlap effect and (ii) the presence of a conductive gel layer. We propose a method which integrates both effects by assuming a simple model for the conductive gel at the pore wall. We observed the following three domains of pore size: (i) r0 > 70 nm, where surface effects are negligible; (ii) approximately 17 nm < r0 < 70 nm, where the pore/solution interface could be described as a conductive gel of thickness around 1 nm; (iii) r0 < approximately 17 nm, which corresponds to the region strongly damaged by the ion beam and is not analyzed here. The first one (zeta = -36.2 mV) corresponds to the raw material when etching has completely removed the ion beam predamaged region, which corresponds to the second intermediate domain (zeta = -47.3 mV). There the conductance of the gel layer deduced from the treatment of streaming potential data was found to be compatible with the number of ionic sites independently determined by the electron spin resonance technique.