High-level coupled cluster calculations obtained with the Feller-Peterson-Dixon (FPD) approach and new data from the most recent version of the Active Thermochemical Tables (ATcT) are used to reassess the enthalpy of formation of gas-phase C2H2O4 (oxalic acid). The theoretical value was further calibrated by comparing FPD and ATcT gas-phase enthalpies of formation for H2CO (formaldehyde) and the two low-lying conformations of C2H4O2 (syn and anti acetic acid). The FPD approach produces a theoretical enthalpy of formation of gas-phase oxalic acid of -732.2 +/- 4.0 kJ/mol at 298.15 K (-721.8 +/- 4.0 kJ/mol at 0 K). An independently obtained ATcT value, based on reassessing the existent experimental determinations and expanding the resulting thermochemical network with select mid-level composite theoretical results, disagrees with several earlier recommendations that were based solely on experimental determinations but is in excellent accord with the current FPD value. The inclusion of the latter in the most recent ATcT thermochemical network produces a further refined value for the gas-phase enthalpy of formation, -731.6 +/- 1.2 kJ/mol at 298.15 K (-721.0 +/- 1.2 kJ/mol at 0 K). The condensed-phase ATcT enthalpy of formation of oxalic acid is -829.7 +/- 0.5 kJ/mol, and the resulting sublimation enthalpy is 98.1 +/- 1.3 kJ/mol, both at 298.15 K.