Abstract: According to the invention a piezoelectric crystal, e.g. a quartz crystal, coated with a metal or another suitable material is exposed to the action of a corrosive liquid, preferably by being immersed therein. The mass change on the crystal surface, caused by the corrosion, and the resulting change in the natural oscillation frequency of the crystal is determined and constitutes a measure of the corrosivity of the liquid. The natural oscillation frequency of the crystal can in this way also be measured in situ in the liquid, in which way the dynamic course of the corrosion reaction can be followed continuously. According to a special embodiment of the invention the sensitivity during the measurement can be increased, if the crystal is pretreated by being activated with a sulphide solution.