An FTIR spectroscopic study of the adsorption of natural and O-18-enriched CO2 gases on MgO powders was performed and was compared with temperature-programmed desorption analysis. Two types of CO3 species (a and b) were found after (CO2)-O-16 adsorption at room temperature : they have characteristic wavenumbers 1659, 1329, and 1024 cm(-1) (type a) and 1626, 1273, and 947 cm(-1) (type b), respectively. Ab initio molecular orbital calculations gave theoretical wave numbers. According to comparison of these numbers, type b is a monodentate model. Type a could not be reproduced in the present limited cluster model. It is inferred that it also comes from a monodentate model with partly bidentate and "tridentate" characters.