Sensor system with a sensor device having a through-flow cell for liquid, a detector device for measuring a property of the liquid in the through-flow cell and for generating an associated detector signal, a sensor control for analyzing the detector signal, wherein the sensor control detects a liquid transition between two different liquids in the through-flow cell when a change (per unit time) in the detector signal is greater is than a threshold value. In the case of such a liquid transition detection, the sensor control generates an alarm signal. Liquid transition detection is carried out by optical, temperature and/or conductivity sensors.