Irradiation of a Ne matrix sample containing NO and CO near 4 K with an ArF excimer laser at 193 nm yielded new lines at 2045.1 and 968.0 cm-1 that were depleted upon secondary photolysis at 308 nm. These lines are assigned to C=O stretching and mixed stretching modes of ONCO, based on results of N15 -, C13 -, and O18 -isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP and PW91PW91/aug-cc-pVTZ) predict five stable isomers of NC O2: ONCO, NCOO, N-cyc-C O2, CNOO, and cyc-CNOO, listed in order of increasing energy. According to B3LYP calculations, ONCO has a trans configuration, with bond angles of ∠ONC≅136.3° and ∠NOC≅160.7°. Calculated vibrational wave numbers, IR intensities, N15 -, C13 -, and O18 -isotopic shifts for ONCO agree satisfactorily with experimental results. ONCO was formed from reaction of CO with NO in its excited state.