An ion-selective electroanalyzer for bio-electrochemically controlled hemodialysis will be described. The bedside analysers are solid-contact flow-through electrodes based on tubular carrier-PVC-membranes for potassium, sodium, calcium and chloride. This multi-measurement system with series-connected biosensors in modular construction is provided with a chemosensor for pH measurements. Besides the technical aspects of the apparatus, the quantitative electrode characteristics and the fundamental basis for the application of ion activities in clinical analytic measurement will be described.