Apparent molar heat capacities and volumes have been determined at T = 298.15 K for aqueous solutions of adipic acid (H(2)A, A = C6H8O4), sodium hydrogen adipate (NaHA), and a buffer solution of NaHA and Na(2)A; and for aqueous solutions of L-tartaric acid (H2T, T = C4H4O6) a buffer solution of H2T and NaHT, a buffer solution of NaHT and Na2T, and a solution of sodium L-tartrate (Na2T). The experimental results have been analysed by using a simple extended Debye-Huckel equation, to derive the partial molar properties of aqueous adipic acid, L-tartaric acid and their corresponding sodium salts at infinite dilution, and the standard partial molar volume and heat capacity changes for the ionization of adipic acid and L-tartaric acid. (C) 2000 Academic Press.