It is found that effective carbonate formation occurs at a clean Si(100) surface between CO2 and surface oxidants : N2O and O-2. The reactions of CO2 with N2O or O-2 are studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. CO2 physisorbed on Si(100) desorbs exclusively at similar to 75 K. However, when CO2 is coadsorbed with N2O or O-2, CO2 is converted very effectively into surface carbonate species below 140 K. The carbonate species are dissociated to produce desorbed species of CO or CO2 in the temperature range 200-600 K. By using (CO2)-O-18, it is confirmed that the carbonate is composed of the adsorbate CO2 and one oxygen atom from the surface oxidant. In the case of N2O/CO2 coadsorption, the carbonate formation is mainly induced by thermal dissociation of N2O adsorbates. In the case of O-2/CO2 coadsorption, the carbonate formation is not only induced by dissociative chemisorption of O-2, but also proceeded with chemisorbed dioxygen that likely adsorbs in a peroxy bridging configuration.