The first and second molal dissociation constants of oxalic acid were measured potentiometrically in a concentration cell fitted with hydrogen electrodes. Measurements were made at six temperatures ranging from 5 degrees C to 125 degrees C at four ionic strengths ranging from 0.1 mol.kg(-1) to 1.0 mol.kg(-1) (NaCl and NaCF3SO3). The second molal dissociation constant was measured in NaCF3SO3 media only. The data obtained in this study were combined with the corresponding literature values, including a number of recalculated Harned cell and concentration cell measurements and previous results from this laboratory. Data for first molal dissociation constant obtained in the two ionic media were regressed simultaneously to yield a five-term equation that describes the dissociation in the anionic form. The thermodynamic properties obtained at 25 degrees C and infinite dilution are : log K-1a(theta) = -1.401 +/- 0.052, Delta H-1a = (-0.7 +/- 0.7) kJ.mol(-1), Delta S-1a = (-29 +/- 2) J.K-1 mol(-1), and Delta C-p,C-1a = (-231 +/- 6) J.K-1 mol(-1). Similar treatment of the original and recalculated data for the second molal dissociation constant yielded a nine-term equation and the following values at 25 degrees C and infinite dilution : log K-2a(theta) = -4.264 +/- 0.014, Delta H-2a = (-7.3 +/- 0.5) kJ.mol(-1), Delta S-2a = (-106 +/- 2) J.K-1 mol(-1), and Delta C-p,C-2a = (-229 +/- 27) J.K-1.mol(-1).