The low-lying electronic states of CCO have been investigated by photoelectron spectroscopy of CCO- al wavelengths of 266 and 355 nm in conjunction with ab initio calculations. Photodetachment is observed to occur to the <(X)over tilde (3) Sigma(-)>, <(A)over tilde (3) Pi>, <(a)over tilde (1) Delta>, anti <(b)over tilde (1) Sigma(+)> electronic states of CCO. This marks the first observation of the low-lying singlet states. A revised value for the electron affinity of CCO is found to be 2.289 +/- 0.018 eV, These results are compared with CASPT2 ab initio calculations of rile energetics and structure of the ground and excited states of CCO and CCO-. Using the measured electron affinity of CCO, the heats of formation Delta(f)H(298)(0)(CCO)=3.99 +/- 0.20 eV and Delta(f)H(298)(0)(CCO-)=1.67 +/- 0.20 eV are determined. In addition, the C-C bond dissociation energies in CCO and CCO- are determined, as well as the H-CCO bond energy in HCCO.