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The first and second molal dissociation quotienls of oxalic acid were measured potenliometrically in a concentration cell filled with hydrogen electrodes. The emf of oxalic acid-bioxalate solutions was measured relative to an HCI standard solution from 25 to 125°C over 25° intervals at nine 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 five-term equation that yielded the following thermodynamic quantities at infinite dilution and 25°C: logKIa = -1.277±0.010, ΔHl̊a = -4.1± 1.1 kJ-mol-1, ΔSl̊a = 38±4 J-K-1 mol-1, and ΔCp̊,la = -168±41 J-K-1-mol-1. Similar measurements of the bioxalateoxalate system were made at 25° intervals from 0 to 175°C at seven ionic strengths from 0.1 to 5.0m. A similar regression of the experimentallyderived and published equilibrium quotients using a seven-term equation yielded the following values at infinite dilution and 25°C: log K2a = -4.275±0.006, ΔH2̊a = -6.8±0.5 kJ-mol-1, ΔS2̊a = -105±2 J-K-1mol-1, and ΔC̊p,2a = -261 ±12 J-K-1-mol-1.