Structures, energies, and harmonic vibrational frequencies of CN2O2 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules and in a study of the mechanism of the reaction between NCO and NO radicals. Nitrosoisocyanate, ONNCO (1), earlier studied as a collision complex in the reaction of NCO and NO (Lin, M. C.; Melius, C. F. J. Phys. Chem. 1993, 97, 9124) is the most energetically favorable CN2O2 isomer, but its 18 kcal/mol unimolecular dissociation barrier is very low. Thus 1 can only be observed as a short-lived intermediate. However, nitrosofulminate, ONCNO (8), and nitryl cyanide, NCNO2 (12), higher energy isomers (69 and 38 kcal/mol above trans-1a, respectively), are more stable than 1 toward decomposition. This offers species 8 and 12 as interesting molecules for experimental study. Moreover, 12 can be a reasonably stable molecule as its C-N bond dissociation energy (59 kcal/mol) and the barrier to decomposition into N-2 and CO2 (54 kcal/mol) are rather high, being comparable to those of nitromethane. The estimated large values of the heat of formation (Delta H degrees(f 300) = 60 kcal/mol) and of the decomposition energy of 12 (12 --> N-2 + CO2; Delta E = 150 kcal/mol) make this species potentially interesting as a high-energy molecule. bur study also includes four- (2) and three-membered (17) cyclic and bicyclic (3) isomers. The C-s cyclic isomers, 2 and 17, are extremely unstable, but the bicyclic C-2v form (3) has a 29 kcal/mol dissociation barrier and should be observable.