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The solubility of gypsum (CaSO4·2H2O) in ammonium solutions plays a significant role to prevent gypsum from scaling on the heater and tower in the treatment of ammonium-N wastewater bearing sulfate ion by steam stripping process.In this work the solubility of CaSO4·2H2O in the solution of NH4Cl and the mixed solution of NH4Cl–(NH4)2SO4 was investigated using dissolution method over a temperature range from 298.15 to 343.15 K.The results show that in the two solutions the solubility of CaSO4·2H2O increases with increasing NH4Cl concentration but the temperature has little effect.In addition,a mixed electrolyte NRTL model was employed to calculate the solubility of CaSO4·2H2O in different concentrations of NH4 Cl at various temperatures.New model parameters for electrolyte NRTL model were obtained via the regression of experimental data.The results show the calculated solubility is in good agreement with experimental values with the relative average deviation of 3%.