Large-scale coupled cluster calculations have been carried out for C5O. The equilibrium geometry is linear with bond lengths (in order of chemical formula) of 1.2802, 1.2928, 1.2622, 1.2840, and 1.1554 Angstrom. The equilibrium dipole moment is predicted to be 4.06 D, with the negative end at the terminal carbon site. The nu(1) band at 2290 cm(-1) has a large IR intensity of 3813 km/mol. Vertical ionization energies of 9.97 and 10.43 eV are predicted for the first sigma and pi ionization, respectively.