A detailed ab initio quantum chemical investigation of the potential energy surface associated with the isomerization reactions of the formaldiminoxy (CH2NO) radical is reported. CH2NO((2)A') is the direct product of the reaction between triplet methylene and "prompt" nitric oxide. The quantum chemical calculations were performed at the Gaussian-2 (G2), CASSCF, CASPT2, and QCISD(T) levels of theory using basis sets that range from cc-pVDZ and cc-pVTZ up to the 6-311+G(3df,2p) of G2. A total of 11 isomers (plus 9 rotamers) and 28 transition states linking them are identified and characterized on the potential energy surfaces. The isomers and their rearrangement reactions are conveniently divided into two classes: those with CNOand NCO-chain backbones. The latter class of molecules are generally more stable, with the NH2CO isomer being the most stable with a predicted heat of formation of -3.6 +/- 1 kcal/mol. Interconversion between the two groups occurs via a cyclic isomer of CH2NO. The energetics of the various isomerization pathways are expected to influence the subsequent dissociation reactions of the formaldiminoxy isomers.