The nature of the reaction products between CoO2 and molecular O-2, isolated in rare gas matrices. have been investigated using IR absorption spectroscopy. In this paper, we report on the vibrational spectrum of the CoO4 molecule in its around and first low-lying excited states. Isotopic substitutions using O-16(2) and O-18(2) precursors. as well as O-16(2) + O-18(2) and O-16(2) + (OO)-O-16-O-18 + O-18(2) mixtures in either excess argon or neon. enable demonstration of C-2v and C-s structures for the respective states. CoO4 is formed following molecular diffusion by complexation of ground-state CoO2 by an O-2 molecule. The molecule is first formed in the excited state and then spontaneously relaxes to the ground state after remaining in the dark. The kinetics of relaxation can be fitted to a first-order exponential decay with all excited-state lifetime estimated around 23 +/- 2 min in argon and 15 +/- 2 min in neon, indicative of a slow, spin-forbidden process. Population of the excited state is induced by photons around 4250 +/- 250 cm(-1). Experimental results are compared to density functional theory (DFT) calculations at the BPW91/6-311G(3df) level. Electronic and geometrical optimizations were carried out starting from the ground-state precursors (i.e., (3)Sigma(g)(-) for O-2 and (2)Sigma(g)(+) for CoO2). Calculations predict a (2)A(2) (C-2v) ground state and a (4)A' (C-s) first excited state 0.37 eV above, close to the 4250 +/- 250 cm(-1) experimental excitation energy. The transition pathway is found to involve two supplementary states with crossed potential energy surfaces (PESs): a B-2(1) excited state. 0.48 eV above the ground state. reached first through all adiabatic transition with a photon around 4800 cm(-1), and a B-4(1) transition state into which the system relaxes before finally attaining the (4)A' (C-s) excited state. Harmonic frequencies and absolute intensities are also calculated and compared with the experimental data, indicating however that the DFT underestimates the internuclear distances for both configurations. Force and interaction constants were obtained with a semiempirical harmonic force-field potential calculation. They were then used in an empirical rule of plot linking force constants and internuclear distances in order to obtain an estimate of the Co-O bond lengths for each state and are compared to the DFT predictions.