An electronic absorption spectrum of CCO -isolated in a neon matrix has been observed. The matrix was prepared by co-deposition of the mass-selected anions with excess neon at 6 K. The observed absorption spectrum is assigned to the A 2 Σ + r X 2 Π electronic transition of linear CCO -on the basis of mass-selection, photobleaching behavior, comparison with ab initio calculations, and the discernible vibrational structure. The origin band is located at 817.4(2) nm and the fundamental frequencies in the A 2 Σ + state are ν 1 (σ + ) ) 2082(3), ν 2 (π) ) 656(3), and ν 3 (σ + ) ) 1185(3) cm -1 . The ν 1 fundamental in the electronic ground state has been observed at 1876.7(1.0) in the infrared spectrum.