Heat capacities of aqueous solutions of Na2SO4 and Na2CO3 up to near saturation (1.9 and 2.5 mol(.)kg(-1), respectively) and of NaOH (to 7 mol(.)kg(-1)) have been measured at 25 degreesC with a Picker flow calorimeter. The calorimeter performance was checked using concentrated NaCl(aq) solutions. On the basis of these measurements, an experimental protocol suitable for the reliable determination of the heat capacities of concentrated electrolyte solutions by Picker calorimetry was established. The heat capacities for Na2SO4(aq), Na2CO3(aq), and NaOH(aq) and literature data for the apparent molar volumes of NaOH(aq) at 25 degreesC were correlated using the Pitzer formalism. A number of inadequacies in previous models at high concentrations and for extrapolation to infinite dilution are discussed. In particular, it has been confirmed that the heat capacity data for Na2CO3(aq) at low concentrations must be corrected for the hydrolysis of the carbonate ion. Standard partial molar heat capacities for the three salts and the standard partial molar volume of NaOH(aq) have been derived at 25 degreesC.