Having measured δ18O of firn air CO2 for three Antarctic sites (Dronning Maud Land, Dome Concordia, and Berkner Island), we established that at depth the δ18O(CO2) values are systematically shifted away from atmospheric values toward isotope equilibrium with H2O from precipitation (firn matrix). The δ18O(CO2) shifts increase with depth and thus with CO2 age. The equation of a first-order reaction is used to describe and calculate CO2 oxygen isotope exchange rates. The calculated exchange rates clearly show a correlation with firn temperatures. Assuming that the same mechanism operates at different sites and at different temperatures, a pseudoactivation energy of 7.8 ¿ 0.8 kcal/mol is estimated. The occurrence of exchange means that firn is not chemically inert in that sense that some form of CO2 hydration takes place. Possible mechanisms of CO2 oxygen isotope exchange are discussed and liquid-like layers on firn grains are the most likely interface where this exchange takes place. Other gases (e.g., COS) may likewise be sensitive to hydration or related reactions and this may reduce their integrity in firn. A formal translation of the reaction rates of CO2 oxygen isotope exchange in firn to a COS hydration rate appears to indicate COS preservation on the scale of tens of thousands of years. We suggest that the value of CO2 oxygen exchange rate in firn, expressed as half-reaction time T$^{1}\!big/\!_{2}$, may be used as a new firn characteristic, giving a measure of hydration activity in firn, an indirect measure of firn metamorphism and be correlated with other firn processes near a pore close-off-depth. T$^{1}\!big/\!_{2}$ values may possibly correlate with the process of He and Ne diffusion/expulsion during the close-off of pores in firn, as well as the release of 14CO produced in situ in the firn matrix.