LaO(2) was produced in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio electronic structure calculations. The calculations included density functional theory, second-order perturbation theory, coupled cluster theory, and complete active space self-consistent field methods. The adiabatic ionization energy of the molecule and vibrational frequencies of the molecule and its cation were measured accurately for the first time from the MATI spectrum. Numerous ionization processes of lanthanum dioxide, peroxide, and superoxide were considered; the (3)B(2) ← (4)B(2) electronic transition of the dioxide was assigned upon comparison with the observed spectrum. The ionization energy and O-La-O bending frequency of the (4)B(2) neutral state are 4.9760 (6) eV and 92 cm(-1), respectively. The La-O stretching and O-La-O bending frequencies of the (3)B(2) cationic state are 656 and 122 cm(-1), respectively. The (4)B(2) state is formed by two electron transfer from lanthanum to oxygen atoms, and the (3)B(2) state is produced by the further removal of a lanthanum 6s-based electron.