Flow systems for precise and accurate coulometric determinations of ions that were developed on the basis of electrolytic ion transfer at the aqueouslorganic solution (W vertical bar O) interface are reviewed. The electrolysis cell in the system is composed of a porous poly(tetrafluoroethylene) tube (1.0 mm inner diameter), a metal wire (0.8 mm diameter) inserted into the tube, O into which the tube is immersed, a reference electrode inO and a platinum wire counter electrode in O. The electrolysis is carried out by forcing W containing a species of interest to flow through the narrow gap between the tube and the metal wire. The coulometric determination can be performed with an efficiency of more than 99% and a precision of better than 0.2% based on the ion transfer under an optimum condition, even if the ion is redox inert such as Na+, K+, Mg2+, Ca2+, CIO4-, picrate or alkylsulfonates. The system can be applied to selective electrolytic solvent extraction of ions.