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The first and second molal dissociation quotients of malonic acid were measured potentiometrically in a concentration cell fitted with hydrogen electrodes. The hydrogen ion molality of malonic acidJbimalonate solutions was measured relative to a standard aqueous HCI solution from 0 to 100°C over 25° intervals at five ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The molal dissociation quotients and available literature data were treated in the all anionic form by a seven-term equation. This treatment yielded the following thermodynamic quantities for the first acid dissociation equilibrium at 25°C: log K1a = -2.852 ± 0.003. ΔH1̊a = 0.1 ±0.3 kJ-mol-1. ΔS1̊a = -54.4±1.0 J-mol-1-K-1 and ΔCp̊,1a = -185±20 J-mol-1-K-1. Measurements of the bimalonatelmalonate system were made over the same intervals of temperature and ionic strength. A similar regression of the present and previously published equilibrium quotients using a seven- term equation yielded the following values for the second acid dissociation equilibrium at 25°C: log K2a = -5.697 ± 0.001. ΔH2̊a = -5.13±0.11 kJ-mol-1, ΔS2̊a = -126.3±0.4 J-mol-1-K-1. and ΔCp̊,2a = -250+10 J-mol-1-K-1.