Experimental results, such as NO 2 hydrolysis and the hypergolicity of hydrazine/nitrogen tetroxide pair, have been interpreted in terms of NO 2 dimers. Such interpretations are complicated by the possibility of several forms for the dimer: symmetric N 2O 4, cis-ONO-NO 2, and trans-ONO-NO 2. Quantum mechanical (QM) studies of these systems are complicated by the large resonance energy in NO 2 which changes differently for each dimer and changes dramatically as bonds are formed and broken. As a result, none of the standard methods for QM are uniformly reliable. We report here studies of these systems using density functional theory (B3LYP) and several ab initio methods (MP2, CCSD(T), and GVB-RCI). At RCCSD(T)/CBS level, the enthalpic barrier to form cis-ONO-NO 2 is 1.9 kcal/mol, whereas the enthalpic barrier to form trans-ONO-NO 2 is 13.2 kcal/mol, in agreement with the GVB-RCI result. However, to form symmetric N 2O 4, RCCSD(T) gives an unphysical barrier due to the wrong asymptotic behavior of its reference function at the dissociation limit, whereas GVB-RCI shows no barrier for such a recombination. The difference of barrier heights in these three recombination reactions can be rationalized in terms of the amount of B 2 excitation involved in the bond formation process. We find that the enthalpic barrier for N 2O 4 isomerizing to trans-ONO-NO 2 is 43.9 kcal/mol, ruling out the possibility of such an isomerization playing a significant role in gas-phase hydrolysis of NO 2. A much more favored path is to form cis-ONO-NO 2 first then convert to trans-ONO-NO 2 with a 2.4 kcal/mol enthalpic barrier. We also propose that the isotopic oxygen exchange in NO 2 gas is possibly via the formation of trans-ONO-NO 2 followed by ON + migration.