Abstract Surface concentration polarization induces fouling processes accompanied by changes in the resistance of a semipermeable membrane, but the connection between these two phenomena has not yet been evaluated. Here, we propose to connect them by introducing the concept of pH equivalent (pHeq), which is used to describe the electrical properties of a polarized membrane surface in terms of a pH value at which a membrane is conditioned. First, the resistance and capacitance of polyamide thin-film membranes conditioned at different pH were evaluated ex situ using electrical impedance spectroscopy (EIS), which decreases and increases in proportion with increasing pH when the pH exceeds the pKa of the ionization of carboxyl groups. Filtration of 0.1 mM Fe(III) chloride solution in dead-end mode was then conducted as a model reaction for the fouling process. The occurrence of concentration polarization in the early stage indeed led the capacitance and the resistance at the membrane/electrolyte interface to a pHeq similar to that of an alkaline-like environment, while no significant decline in flux or rejection was observed. In addition to being responsible for the formation of Fe(OH)3 in fouling, this high pHeq is also likely the bridge connecting the electrical characteristics of the membrane to macroscopic observations (flux and rejection) based on the electroviscous consideration.