Abstract: Cyanomethylene radical (HCCN) is an important intermediate in the nitrile chemistry in both the earth's and the Titan's atmosphere. Despite that the mechanism for the oxidation of HCCN has been already computationally explored, the key Criegee intermediate, NCC(H)OO, remains unobserved yet. By photolyzing mixtures (1:50:1000) of either HC(N_2)CN/O_2/N_2 (266 nm) or HCCNCO/O_2/N_2 (193 nm) at 15.0 K, the elusive carbonyl oxides NCC(H)OO, in syn- and anti-conformations, have been generated and characterized with IR spectroscopy. The spectroscopic identification is supported by ¹⁸O-labeling experiments and the quantum chemical calculations at the BP86/6-311++G(3df, 3pd) level. Upon subsequent UV-light irradiation, both conformers of NCC(H)OO further react with O_2 and yield NCC(O)H and O_3, whereas, the dioxirane isomer HC(O_2)CN, which is lower than syn-NCC(H)OO by 23.7 kcal/mol at the CCSD(T)-F12a/aug-cc-pVTZ//BP86/6-311++G(3df, 3pd) level, was not observed experimentally.