For measuring the water vapor dew point in gases, a moisture-dependent electrical quantity of a sensor surface suitable for indicating formation of dew droplets is held by temperature control at a desired value corresponding to a stable dew mass. The temperature of the sensor surface corresponding to the desired value of the moisture-dependent electrical quantity is measured as dew point temperature. For compensating the influence of soiling, from time to time the desired value of the moisture-dependent electrical quantity is set in that the temperature of the sensor surface is lowered from a value lying above the dew point temperature and a periodic time temperature variation superimposed on the lowering. If the sensor surface is soiled, periodic time variations of the moisture-dependent electrical quantity occur at the same time but they become a monotonic variation when the dew point temperature is reached. The value of the moisture-dependent electrical quantity measured on the transition of the periodic time variations to a monotonic variation is then used as the reference or desired value which is maintained by the control of the temperature of the sensor surface. Furthermore, the control parameters for controlling the moisture-dependent electrical quantity are continuously corrected on the basis of an oscillation analysis.