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The influence of Fe3+as an example of an impurity on the crystallization of ammonium sulfate as an example of an inorganic compound is discussed based on theory and experiments. The metastable zone width was increased up to 4.5 times by 100 ppm of impurity but the effect seems to level out at higher Fe3+concentrations. Fe3+also proves to be a very effective growth and dissolution rate suppressor. The achieved data were analyzed and described by the use of the model of Kubota–Mullin. A growth stoppage could be observed at 86.5 ppm. The effect on the dissolution rate is less pronounced. This growth suppressive behavior was not consistent for all of the individual faces of ammonium sulfate. As a consequence a change in the crystal morphology could be observed. With increasing Fe3+concentrations the crystal habit changes from cubic to a needle like shape. [ABSTRACT FROM AUTHOR]