Water activities of the aqueous ternary systems containing three amino acids glycine, D-alanine or L-valine, and magnesium sulfate, have been obtained at 298.15 K using the isopiestic method. From the. water activity data, activity coefficients of the amino acids in these systems were calculated with an equation in terms of the solute molalities. The effect of MgSO4 on the activity coefficient of the amino acids was clarified. The results show that the presence of MgSO4 has a significant effect on the activity coefficient of the amino acids and this effect is different for them. A combination of two models Pitzer-Debye-Huckel and modified Wilson local composition was applied to correlate the activity coefficient data. (C) 2011 Elsevier B.V. All rights reserved.