Apparent molar volumes Vφand heat capacities Cp , φof aqueous solutions of acetic acid (CH3CO2H) and sodium acetate (CH3CO2Na) , and Cp , φfor hydrochloric acid (HCl) and sodium hydroxide (NaOH) were determined at temperatures from T = 278.15 K to T = 393.15 K at the pressure 0.35MPa. Apparent molar volumes were obtained from density measurements using an Anton Paar vibrating-tube densimeter (DMA 512, Anton Paar, Austria). Heat capacities were obtained using a fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC model 5100, Calorimetry Sciences Corporation, Provo, UT). Infinite dilution partial molar volumes Voand heat capacities Cpowere obtained over the range of temperatures by extrapolation to m = 0 of the fitted surfaces (Yφ, T , m). The volume results were combined with values of Vofrom the literature for aqueous NaCl, HCl, and NaOH to obtain ΔrVmofor ionization of acetic acid. Measured values of Cp , φwere extrapolated to m = 0 for HCl(aq) and NaOH(aq) and used to obtain values of ΔrCp , mofor ionization of CH3CO2H(aq). The values of ΔrVmoand ΔrCp , moallow the determination by integration of the equilibrium constant Kaand other thermodynamic properties for ionization of CH3CO2H(aq) from T = 278.15 K to T = 393.15 K at the pressure 0.35MPa.