The influence of two typical ionic impurities (Fe2+ and SO42-) and operating factors in the presence of these impurities on phosphoric acid hemihydrate (H3PO4 center dot 0.5H(2)O) crystallization is investigated. The crystal growth mechanism and ion-crystal face interactions are systematically studied using an in-line focused beam reflectance monitoring (FBRM) technique and molecular dynamic simulation. It is noted that the seeding supercooling and cooling rate during crystallization have a more significant effect on the crystallization process in the presence of Fe2+ than that of SO42-. From microscopic observation of crystals, it is clear that Fe2+ has a notable effect on crystal morphology, while SO42- has very little effect. In order to obtain a deep understanding of the growth mechanism, a molecular simulation study using Cerius(2) software was performed. The absolute values of the calculated binding energies for Fe2+ are larger than that for SO42-. Significant differences of Fe2+ binding energies to the different crystal faces are found, while there is little difference for the case of SO42-. Therefore, it is concluded from the simulation that Fe2+ inhibits the growth of some crystal faces, but SO42- has no significant inhibition to a particular crystal surface, which is in good agreement with the experimental observation. Understanding these effects is very helpful to aid optimization and improve process control.