Structures and energies of CN2O3 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules. The most energetically favorable isomer, five-membered cyclic nitrous carbonate, has only a 16 kcal/mol dissociation barrier toward decomposition into CO2 and N2O. Nitroisocyanate, O2N-NCO, a collision complex in the gas-phase reaction of NCO and NO2 radicals, is 29 kcal/mol higher in energy than nitrous carbonate and has a 11 kcal/mol barrier in the exothermic decomposition into CO2 and N2O, which involves formation of an intermediate four-membered cyclic form. Nitrofulminate has a higher CN bond dissociation energy and is expected to be a more stable molecule than the earlier studied nitrosofulminate (Korkin, A. A.; et al. J. Phys. Chem. 1996, 100, 19840). Regarding its high exothermicity in decomposition (O2N-CNO --> CO2 + N-2 + 1/2O(2); Delta E = -165 kcal/mol), nitrofulminate is suggested as a potential energetic oxidizer. Another candidate is its six-membered cyclic trimer, 2,4,6-trinitro-1,2,3-triazine 1,2,3-trioxide. The estimated gaseous heats of formations of nitrofulminate and trinitrotriazine trioxide are 71 and 108 kcal/mol, respectively. Trinitrotriazine trioxide and two other cyclic trimers, trinitroisocyanurate and 2,4,6-tri(nitrosooxy)-1,3,5-triazine, have been optimized at the HF/6-31G* and at the MBPT(2)/6-31G* levels and characterized by HF analytical harmonic frequencies. Another CN2O3 isomer, dinitrosocarbonyl, O=C(NO)(2), has a low stability toward decomposition into CO and NO radicals.