The two lowest electronic states of the lithium superoxide radical, LiO 2, have been investigated using ab initio theoretical techniques, including RHF SCF, CISD, Davidson-corrected CISD [CISD + (Q)], UHF SCF, UMP2, UMP3, UMP4(SDTQ), spin-projected UHF and UMP, valence and extravalence CASSCF (CASSCF-v and CASSCF-π), and CISD based on CASSCF natural orbitals (CISD-π). Four basis sets ranging in quality from Li (9s4p/5s2p), O (9s5pld/5s3pld) to Li (10s5pld/6s4pld), O(11s7p2dlf/6s5p2dlf) were employed, these being designated TZP, QZ2P, QZ2P+R, and QZ2P+R+f. The investigation encompassed dissociation energies, relative energies of various conformations, geometrical structures, vibrational frequencies, infrared and Raman intensities, dipole moments, cubic force fields, vibration-rotation interaction constants, and symmetry breaking phenomena. The onset of spatial symmetry breaking in the electronic orbitals of the TZP RHF reference wavefunction for X 2A 2 LiO 2 leads to an irremovable singularity in the quadratic force constant for antisymmetric LiO stretching at the isosceles-triangle (C 2v) geometry d(OO) = 1.3266 Å and r(LiO) = 1.7737 Å. This anomalous lowering of spatial symmetry from C 2v to C s, makes the two oxygen atoms inequivalent, and thus it becomes necessary to avert the symmetry dilemma in the reference wavefunction to provide unequivocal evidence for a C 2v geometrical structure of X 2A 2 LiO 2 which is consistent with the ionic model. This task is achieved with the CASSCF-π and CISD-π wavefunctions, the latter yielding d(OO) = 1.3405 Å, r(LiO) = 1.7937 Å, ω 1(a 1) = 1263 cm −1, ω 2(a 1) = 740 cm −1, and ω 3(b 2) = 519 cm −1 at the C 2v equilibrium geometry. Final proposals of d(OO) = 1.335 Å, r(LiO) = 1.76 Å, and D 0(LiO 2) = 62 kcal/mol are made for X 2A 2 LiO 2, as indicated by appurtenant studies of X 2Π gO 2 −1 and X 2Π LiO. Improved predictions are thereby provided for gas-phase LiO, viz., r e = 1.694 Å ω e = 814 cm −1, ν 0 = 798 cm −1, and D 0(LiO) = 86 kcal/mol. The A 2B 2 state of LiO 2 is also predicted to have a C 2v geometrical structure, d(OO) = 1.3497 Å and r(LiO) = 1.8612 Å being obtained at the TZP CISD level, and a final adiabatic excitation energy of T e = 16.7 kcal/mol is determined. A large differential correlation energy effect is found to be important in obtaining an accurate relative energy for the 2Π, C 2∞ v, LiOO linear structure, which is found to be a shallow minimum 22.2 kcal/mol above the X 2A 2, C 2v state with TZP CISD bond lengths of d(OO) = 1.3149 Å and r(LiO) = 1.6341 Å. Finally, 2Π LiO 2 is connected to the X 2A 2 and A 2B 2 minima via two transition states of 2A″ and 2A′ symmetry, respectively, with interconversion barriers near 1.4 and 1.1 kcal/mol.