FT-IR spectroscopic studies of the relaxation of binary solid nonequilibrium mixtures of CO2 and Ar or Kr (1:1000 molar ratio) are reported. The samples were prepared by embedding carbon dioxide in solid rare gas, either after thermal effusion or after adiabatic expansion. The resulting matrices then contained exclusively isolated monomer CO2 or both monomer and aggregate species, respectively. These two nonequilibrium systems were subjected td appropriate thermal treatment, which initiated a stepwise transition to the same final-state, namely, the equilibrium solid, The relaxation paths traced by monitoring the asymmetric stretching vibration of (CO2)-C-13 in natural abundance are, however, fundamentally different. They include an intermediate amorphous and a CO2 cluster phase, respectively. The studies were preferentially carried out in Kr matrix, since the initial spectrum indicates the presence of only one substitutional site for the CO;! monomer, whereas in Ar there are four sites. To facilitate the interpretation of the IR spectra, ab initio calculations were performed on small CO2 clusters.