Density functional theory B3LYP/6-31G(**) and ab initio MP2/6-31G(**) and MP4(SDQ)/6-311++G(**) calculations were carried out to study the structures and isomerization and decomposition mechanisms of oxalic acid. The B3LYP structures and relative energies of the rotational isomers of oxalic acid are found very similar to MP2 results, confirming that the most stable isomer is the doubly intramolecular hydrogen-bonded C-2h structure E1, with four other planar isomers within 6 kcal/mol. It is predicted that unimolecular formation of carbon dioxide and dihydroxycarbene (DHC) from oxalic acid has an activation barrier of 31 kcal/mol and that unimolecular formation of HCOOH from DHC has an activation barrier about 31 kcal/mol higher. The unimolecular formation of CO2, CO and H2O from oxalic acid via a concerted transition state has an activation barrier of only 42 kcal/mol, indicating it is a more favorable unimolecular decomposition channel. On the other hand, hydrogen migration from oxygen to carbon of DHC to produce HCOOH can be accomplished through a hydrogen exchange with H2O (a model for oxalic acid) with an activation barrier of less than 37 kcal/mol. Transition state theory calculations indicate that this bimolecular channel might be responsible for the rapid formation of CO2 and HCOOH in gas phase oxalic acid thermal decomposition, thus confirming the proposal of Beck and Redington. With increasing temperature the unimolecular channel to produce CO2, CO, and H2O might become significant.