The metastable zone width (MZW) is a vital parameter for the design of industrial crystallization processes of inorganic salts and could be affected by the addition of impurities. Here, the effect of the trivalent ions Fe3+, Cr3+, and Al3+ on the MZW of ammonium sulfate was observed. All the experiments were performed with the help of ultrasonic speed measurements. Impurity concentrations up to 100 ppm at a solution pH of 4 increase the MZW depending on the metal ion. The pH value also showed a major influence on the effect of trivalent metal impurities. The results were compared to the distribution of metal-aquo-complexes existing in aqueous solutions contaminated by trivalent metal ions. The distribution was determined by using the computer-based simulation software Hyss. The first hydrolysis product was identified to be active in widening the MZW.