The reactivity of atomic cobalt toward molecular oxygen in rare gas matrices has been reinvestigated. Experiments confirm that Co atoms in their a(4)F around state are inert toward O-2 in solid argon and neon but reactive in the b(4)F first excited state, in agreement with the previous gas-phase study of Honma and co-workers. The formation of CoO2 starting from effusive beams of Co and O-2 has been followed by IR absorption spectroscopy, both in neon and argon matrices. Our observations show that only the dioxo form, OCoO. is stabilized in the matrix and that IR absorptions previously assigned to the peroxo and Superoxo forms are do,to other, larger species. The present data strongly support the linear geometry in rare gas matrices proposed by Weltner and co-workers. We report on measurements on all IR-active fundamental modes for (OCoO)-O-16-O-16. (OCoO)-O-18-O-18, and (OCoO)-O-16-O-18 with additional combination transitions supplying anharmonicity correction. This allows for a 5.93 +/- 0.02 mdyne/Angstrom CoO harmonic bond force constant in solid neon. Using the empirical relationship previously optimized for the CoO diatomics. an approximate value for the CoO internuclear bond distance is proposed (1.615 +/- 0.01 Angstrom). In light of recent theoretical studies predicting (2)A(1) or (6)A(1) electronic Ground states, the geometry and electronic structure of the OCoO molecule has also been reconsidered. Calculations carried out at the CCSD(T)/6-311 G(3df) level indicate a linear structure with an r(2) = 1.62 Angstrom bond distance, consistent with the experimental estimate. For later studies of larger systems. where CCSD(T) calculations become too time-consuming, an effective DFT-based method is proposed which reproduces the basic electronic and geometrical properties of cobalt dioxide. Quantitative results are compared to the experimental data and high-level results regarding bond length and frequencies. This DFT method is used to propose a reaction pathway.