The solubility of gypsum (CaSO4 center dot 2H(2)O) in ammonium solutions plays a significant role to prevent gypsum scaling on the heater and tower in the treatment of ammonium-N wastewater bearing sulfate ions by the steam stripping process. In this work solubilities of calcium sulfate dihydrate in NH4Cl, NH4NO3, and mixed NH4Cl and (NH4)(2)SO4 solutions up to 343.15 K were measured using the classic isothermal dissolution method. The investigated concentration (at ambient temperature) is up to 1.50 mol.dm(-3) for both NH4Cl and NH4NO3. The solubility of CaSO4 center dot 2H(2)O was found to increase sharply with either NH4Cl or NH4NO3 concentration, whereas the temperature has a limited effect. The XRD analysis of equilibrated solids for these systems shows that CaSO4 center dot 2H(2)O is stable in all cases over the temperature range (298.15 to 343.15) K. The electrolyte nonrandom two-liquid (the electrolyte NRTL) model embedded in AspenPlus was applied to model the solubility of CaSO4 center dot 2H(2)O in the above systems. The newly obtained model parameters were used to well estimate the solubility of CaSO4 center dot 2H(2)O in all cases with a relative deviation of 1.52 %.