Rotational transitions of the carbon-chain radicals, NC2O and NC3O, have been observed for the first time by Fourie-transform microwave spectroscopy. The radicals were produced in a supersonic free jet by a pulsed discharge of acetylcyanide (CO(CH3)(CN)) diluted in Ar for NC2O or a gas mixture of HC3N and O-2 diluted in Ar for NC3O. Pure rotational transitions from N = 1-0 to N = 3-2 for NC2O and those from N = 2-1 to N = 6-5 for NC3O, both with fine and hyperfine structures, were observed. The rotational, spin-rotation, and hyperfine coupling constants for the radicals have been precisely determined. Ab initio calculations at the RCCSD(T) level of theory considering the correlation of core electrons using the cc-pCVTZ basis sets have been performed. The present observations and the ab initio calculations revealed that both the radicals have bent structures in the ground states. (C) 2004 Elsevier B.V. All rights reserved.