The results of an ab initio quantum chemical study, carried out at the Gaussian-2 (G2) level of theory, of the potential energy surface governing the isomerization and dissociation reactions of the CHNO isomers (to CH + NO, NH + CO, and OH + CN) are reported. In general, several pathways were found that interconnect each open-chain or cyclic isomer with the others. In particular, two bifurcation points were located; the first connects HCNO to both HNCO and cyclic N(H)CO, the second connects HOCN to HNCO and cyclic N(H)CO. The lowest energy pathways for the isomerization of the open-chain species are two-step reactions that proceed via cyclic intermediates. Comparison of the G2 energies with those obtained in density functional (DFT) and QCISD(T)/cc-pVTZ calculations show a reasonable level of consistency, although differences up to similar to 9 kcal/mol were found between G2 and DFT.