In mass crystallization the size distribution of the crystals produced is determined by the rate of growth and nucleation. Both rates are functions of the supersaturation present in the system. Usually a low nucleation rate is preferred for producing large crystals, but this requires low production rates. Fines removal is an alternative means of achieving the desired coarse product. It is applied in the industrial production of, for example, potassium chloride or ammonium sulfate. In this contribution the effect of fines removal, as well as the role of nucleation kinetics, has been investigated by dynamic simulation. The model used for simulation consists of the solute mass balance which is coupled by kinetics to the population balance of the crystals. The calculations show that true quantitative results cannot be obtained with simple models. Nevertheless, it is possible to investigate the dynamic behaviour of crystal size distributions. It is also shown how the intensity of fines removal and the presumed nucleation kinetics affect the stability of crystallizers.