A detector for open-tubular column liquid chromatography based on an ion-selective microelectrode cell assembly is described. The microelectrode with tip diameter of approximately 1 μm is directly inserted into the downstream column end. Estimated detector cell volumes are about 10−14 l (25-μm i.d. column). Experimental evidence is presented that proves the cell volume to be smaller than 5 × 10−10 l. The detection system should be adequate for open-tubular columns with an inner diameter ≥ 5 μm.