The experimental data of metastable phase equilibrium of K+
Na+
Mg2+//Cl-
SO42--H2O quinary system at the conditions of temperature 15± 0.1℃
air flowing velocity 3.5-4.0m/s
air relative humidity 35-40%
and evaporation rate 2-4 mm/24h is presented. From these metastable equilibrium data
a metastable phase diagram is drawn. It consists of eight regions of crystallization saturated with sodium chloride: potassium chloride
glasserite
schoenite
carnallite
thenardite epsomite
magnesium sulfate hexahydrate
and bischofit. A model of the K+
Na+
Mg2+ // Cl-
SO42--H2O quinary system is deve theloped for calculating the metastable phase equilibrium. This model utilizes activity coefficient expressions of Harvie and Weare. The calculated data of metastable phase equilibrium fit well with the experimental data.