The existing methods for the measurement of supersaturation can be generalized insofar that physical proper-ties are used that show a dependence on concentration of the measurands for supersaturation. The influence of impurities, foreign particles, or ions on the metastable zone width and on the kinetics of nucleation and crystal growth cannot be detected by most of the described methods. Thus, it is necessary to develop a supersaturation sensor which considers the actual crystallization process itself in its measurement method. The idea of that is to induce crystallization on the surface of the sensor by generating an additional supersaturation by cooling and to observe the time-dependent development of the formation of solid matter on the surface which leads finally to incrustation. Assuming a constant cooling rate and constant properties of the sensor surface, the starting time of the incrustation on the sensor surface depends only on the prevailing supersaturation in the process solution. Experimental results obtained for inorganic (KNO3) and organic (adipic acid) crystallizing solutes proved the working of the new sensor.