Abstract This paper presents an example of the use of a turbidity multi-wavelength sensor for an in-line study of fragmentation of crystals in an agitated vessel. Thanks to this in situ sensor, it is possible to continuously record the turbidity spectrum of a suspension. Particle size distributions can be calculated for grain diameters between 0.2 and 10 μm. This sizing technique is applied here to the fragmentation of potassium sulphate in ethanol and in its aqueous solution. A qualitative and quantitative study of the phenomenon of fragmentation can be thus performed. Determination of the influence of mass and stirring rate gives an indication of the prevailing fragmentation mechanisms. The large discrepancies observed between the behaviors in ethanol and in water are due to agglomeration.