For measuring concentrations of fluid-soluble gases with improved drift stability and low production costs, thus dispensing with tedious calibration and/or drift correction routines and re-membraning procedures, a sensor and a system are provided, comprising at least two electrodes, which are covered by sensor fluid at at least one detection site; and an ion-balancing means (50), for example a mixed-bed ion-exchange resin, in contact with the sensor fluid for removing polluting ions.