The crystallization of sodium perborate tetrahydrate (SPT) was carried out in a batch crystallizer with and without seeding. SPT concentration and CSD were monitored as functions of time, and the final crystals visualized by SEM micrography. Experimental evidence for primary and secondary nucleation, crystal growth and crystal agglomeration was observed. Thus, a simulation model including these processes is presented. From the experimental data, seven parameters of the model were fitted by optimization in two different steps.