We report the measurement of the photoabsorption spectrum of Li3O using resonance-enhanced two-photon ionization spectroscopy in the energy range between 0.7 and 2.75 eV. Ab initio geometry optimization calculations at the CCSD(T)/6-311+G(d) level of theory are carried out, resulting in a stable D-3h ground state symmetry for Li3O. Vertical excitation energies are computed from the CCSD(T) potential, and the flatness of the potential energy surface is analyzed. A comparison of the recorded absorption spectrum with the theoretical predictions allows an assignment of all the observed bands and excited states in terms of a D-3h ground state structure. It is argued that the width of the bands is governed by the flat-bottomed shape of the potential energy surface. (C) 2003 American Institute of Physics.