Using a water activity instrument meter, water activity in aqueous solutions Of DL-alanine, glycine, or L-serine, with potassium chloride, molality ranging from 0.0 to 3.0, has been measured at 298.15 K. The reliability of the method was checked comparing the experimental data with literature values. The method proved to be accurate, and the water activities measured for water + amino acid systems are reproducible when compared to the data reported using the isopiestic method. Additionally, a simple theoretical approach applied to those binary systems enabled the calculation of unsymmetric molal amino acid activity coefficients in high agreement with the values found using the isopiestic measurements. Finally, the usefulness of the ternary data to extend the capabilities of thermodynamic models to higher salt and amino acid concentrations was briefly discussed.