Various types of detectors have been utilised in electrophoretic separation systems, but conductivity detection provides the obvious choice for detecting ionic substances. This paper describes a new design of conductivity detector utilising a single, on column, sensing electrode. This new design is seen as offering the twin advantages of much higher resolution, than has previously been possible, and simple construction. The detector has been evaluated using separations of sodium and potassium in capillary isotachophoretic and miniaturised planar polymer isotachophoretic separation systems. Good linearity has been found and the limit of detection of sodium calculated to be 0.43 mM.