Method for determining ion concentration or concentration of a substance in a solution by means of an ion selective, field effect transistor or an ion sensitive sensor having an EIS structure. The method comprises steps as follows: at least at a predetermined point in time of a first measuring phase, the potential lying on the electrode is sensed relative to a reference potential; before beginning a sterilization- and/or cleaning phase following the first measuring phase, the last voltage sensed during the first measuring phase is stored; and at the beginning of a second measuring phase following the sterilization- and/or cleaning phase, is the voltage stored during the sterilization phase is applied on the electrode.