Water activity measurements by the isopiestic method have been carried out on the aqueous ternary system of L-alanine + dipotassium oxalate and the aqueous binary system of dipotassium oxalate over a range of temperatures at atmospheric pressure. From these measurements, values of the vapor pressure of solutions were determined. The experimental vapor-liquid equilibria data for the binary system were correlated with the Pitzer equation. The agreement between the correlations and the experimental data is good. A model based on the Pitzer formalism has been employed to correlate the experimental osmotic coefficient data for the ternary system, and the results show that this model can be satisfactorily used.