Phase diagrams are important tools for design of fractional crystallization processes. However, besides that metastable behaviors are typically found, evaporative crystallization processes are often operated at nonequilibrium with conditions due to high evaporation rates. In such case, the salt-forming regions can be different and more complex than expected based on the equilibrium diagram. Thus in this work, the concepts and principle of salt-forming regions including primary region, extreme region, and conditional region are theoretically proposed to present the complex behaviors of salt-formation. Furthermore, the metastable zones and salt-forming regions are experimentally determined for a typical ternary system of Na+, Mg2+//SO42--H2O at 348 K by the experimental method of isothermal boiling evaporation with an evaporation rate of 2.2 g (L min)(-1) (water). The primary regions (or extreme regions) of Thenardite, Loweite and Sakeite are enlarged and they are 1.16 (1.64), 1.28 (1.34) and 1.33 (1.96) times bigger, respectively, than those in solubility diagram, whereas Vanthoffite' region is reduced. The conditional regions accounts for 26.83% of the diagram's total region, where the salts precipitating maybe one or another or together which depend on the non-thermodynamic conditions, such as crystal seed, evaporation rate, mechanical, and so forth. Thus, knowledge of salt-forming region, especially the conditional region, would be extremely valuable to industry process design and control. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.