On-line measurements of the mean crystal size, the weight percent solids in the product and fines steams, and an estimate of the supersaturation in a 1 m(3) forced-circulation pilot plant crystallizer are performed in this study. From the open loop responses to step changes in the feed, product, recirculation, and fines dissolution flow rates, it is determined that the on-line sensors can provide acceptable measurements ofprocess variables. It is found that the sensors used to measure the weight percent of fines and the supersaturation are sensitive to changes in the brine composition. The open loop responses are also used to design a PI-controller for the control of the mean crystal size. Based on a first order process model of the measured mean size, a cascade PI-controller is designed using the fines dissolution flow rate as the slave controlled variable. At a temperature difference of 15 K between the feed tank and the crystallizer, the controller results in some improvement in the measured mean size of crystals. The controller does not have adequate set point tracking abilities and causes some oscillatory behaviour as a result of disturbances. The controller performance and especially its set point tracking capability was improved when the temperature difference between the feed tank and the crystallizer was reduced to 5 K and further modifications to the crystallizer circuit were incorporated.