Ab initio molecular orbital theory was applied to the HCCO + NO reaction in light of recent experimental studies which point to a predominance of CO plus a (CHNO) isomer as products over CO2 plus HCN. Geometries of stationary points on the [C2HNO2] potential energy surface were optimized using HF/6-31G(d,p) and MP2/6-31G(d,p) calculations, and relative energies were estimated using MP4SDTQ and QCISD(T) calculations and the 6-311G(d,p) basis set and corrected for zero-point energies. Six reaction paths were (partly) characterized, all proceeding through intermediate structures, yielding either CO2 plus HCN or CO plus a (CHNO) isomer. The formation of CO2 is possible by two channels : a three-step process involving cis-nitrosoketene and a four-membered ring as intermediates and a direct fragmentation from formyl isocyanate. The CO formation is possible via four distinct reaction paths. Only one of the latter, involving an aziridine derivative, has an intermediate transition structure lower than those for the CO2-formation paths. The CO-forming channel via formyl isocyanate is entropically favored over the CO2-elimination path. Overall, calculated results are only in agreement with experimental observation if formyl isocyanate is invoked as reaction intermediate. Nevertheless the formation pathway (HCCO + NO --> formyl isocyanate), which is the most exothermic process, could not be identified yet. Structure and properties of the ketenyl radical have also been analyzed.