Apparent molar volumes (V-phi) and heat capacities at constant pressure (CO) of aqueous solutions of sodium oxalate (Na2OX), sodium succinate (Na(2)Suc), Sodium malonate (Na(2)Mal), and potassium oxalate (K(2)Ox) were determined at 25 degrees C up to their saturation limits using vibrating tube densitometry and flow calorimetry. These data were fitted using the Redlich-Rosenfeld-Meyer and Pitzer models. It was established using literature data that V-phi and C-p phi of Na+ and K+ salts with a common anion are additive within well-defined limits even up to high concentrations. This was used to estimate hypothetical values of C-p phi and V-phi at high ionic strengths for the sparingly soluble Na(2)Ox, which are required for the thermodynamic modeling of concentrated electrolyte mixtures such as Bayer process solutions. These values were in reasonable agreement with those estimated using a Pitzer model with parameters for Na2SO4(aq).