A vibrating-tube densimeter (DMA 512P, Anton Paar, Austria) was used to investigate the densities and volumetric properties of aqueous potassium hydrogen phthalate (KHP) and potassium sodium phthalate (KNaP). Measurements were made at molalities m from (0.006 to 0.66) mol . kg(-1), at temperatures from 278.15 K to 368.15 K and at the pressure 0.35 MPa. The densimeter was calibrated through measurements on pure water and on 1.0 mol . kg(-1) NaCl(aq). We also used a twin fixed-cell, power-compensation, differential output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) to measure solution heat capacities. This was accomplished by scanning temperature and comparing the heat capacities of the unknown solutions to the heat capacity of water. Apparent molar volumes V phi and apparent molar heat capacities C-p,C-phi Of the solutions were calculated and fit by regression to equations that describe the surfaces (V-phi, T, m) and (C-p,C-phi, T, m). Standard state partial molar volumes VzO and heat capacities C-p,2(0). were estimated by extrapolation to the m = 0 plane of the fitted surfaces. Previously determined C (Delta C-r(p,m), T, m) for the proton dissociation for HCl(aq) and NaCl(aq) were used to obtain reaction of aqueous hydrogen phthalate. This (Delta C-r(p,m), T, m) surface was created by subtracting C-p,C-phi for KHP(aq) and for NaCl(aq) from the sum of C-p,C-phi for KNaP(aq) and for HCl(aq). Surfaces representing (Delta H-r(m), T, m) and (Delta 8a, T, m), where pe, denotes the molality equilibrium quotient, were created by integration of our (Delta C-r(p,m), T, m) surface using values for (Delta H-r(m), m) and (pK(a), m) at T = 308.15 K from the literature as integration constants.