Extensive experimental investigations into the effect of magnesium and sulfate ionic impurities on cooling crystallization of KCI were carried out in a steady-state Mixed Suspension Mixed Product Removal (MSMPR) crystallizer. The impurities selected for investigation were chosen from those naturally associated with KCI in potash industry. Eighty one experiments were performed in a factorial experimental design scheme. The results exhibited that magnesium ion did not have any significant influence upon crystallizations of KCI, while the sulfate ion enhanced the aggregation rate. The nucleation rate was only slightly affected by the presence of impurities. The size enlargement in presence of sulfate ion was ascribed to an increase in aggregation rate. It was demonstrated that the role of the impurities studied upon KCI crystallization is consistent and predictable and is a function of the chemical character of the impurity species. Based on these results two mechanisms for size enlargement were identified, first due to crystal growth by solid deposition and second due to aggregation. Aggregation was found to be the governing mechanism for producing large crystals.