Hydrometallurgy is a specialized branch of extractive metallurgy dealing with metal recovery from ores, concentrates, and other metallurgical intermediate products in aqueous phase. These processes operate typically at the temperatures between 50 and 250 degrees C, which is lower than that used in pyrometallurgical processes. The knowledge of solubilities of solids and gases in electrolyte systems is required for process development. In this work, H2SO4-FeSO4-H2O and H2SO4-Fe-2(SO4)(3)-H2O systems have been studied for their interest in the zinc leaching process and also for regeneration of pickling acids in stainless steel manufacturing. Database development and Gibbs energy minimization methods for evaluating relevant process systems are discussed. Ternary phase diagrams were compiled with new optimized Pitzer parameters. Development of the databases and calculations presented in this work are the kinds of tools, which may be the basis for controlling the processes in the future.