The present invention relates to a method for separating off a substance from a solution, in which electromagnetic radiation is radiated into the solution, an intensity of the electromagnetic radiation which has been scattered by crystals located in the solution is detected, the detected intensity is compared with a desired intensity (IS) and the temperature of the solution is regulated depending on the difference between the detected intensity and the desired intensity (IS) in such a way that the amount of this difference is reduced. If the amount of the difference between the detected intensity and the desired intensity (IS) is less than a limiting value, a crystallization method is started in which crystals of the substance are obtained which are then separated off.