The charging and discharging processes of conducting polymers, e.g. polypyrrole, are accompanied by the exchange of ions. This effect was used for the development of an electrochemically switchable ion-exchanger for water purification, especially for softening drinking water. The ion-exchange behaviour and capacity of electrochemically and chemically prepared polypyrrole (PPy) in dependence on the incorporated counterions are characterised using an electrochemical quartz crystal microbalance and cyclovoltammetry. An ion-exchanger electrode based on modified PPy-nanoparticles was tested in a laboratory loop. The ion concentrations of the test solutions treated were investigated by atomic absorption spectroscopy and ion-selective electrodes with respect to an application for softening drinking water.