Hydrated CO2.- anion radicals, CO2.-(H2O)(N), are selectively prepared in an electron-impact free jet Of CO2 containing H2O. Mass spectrometric measurement reveals that CO2.-(H2O)(N) reacts with CH3I to form an anion with [(CO2)(CH3I)](-) stoichiometry. The product [(CO2)(CH3I)](-) is further identified as the anion of acetyloxy iodide, CH3CO2I-, based on the observation that [(CO2)(CH3I)](-) photodissociates at 532 nm into CH3CO2- + I or CH3CO2 + I- channels. The CO2.-(H2O)(N) + CH3I reaction thus presents a sharp contrast to the corresponding reaction in solutions: the gas-phase CO2.-(H2O)(N) behaves as a carboxylation reagent for alkyl halides (RX), whereas in aqueous solutions the reaction proceeds as CO2.- + RX --> CO2 + R + X-. Ab intio calculations suggest that CO2.-(H2O)(N) can take on structures preferable for radical reactions: the hydration occurs on the O atoms Of CO2.- and the unpaired electron on the C atom remains uncovered with H2O solvents. The reaction mechanism of the CO2.-(H2O)(N) + CH3I process is discussed in conjunction with previous results Of (CO2)(N)(-) + CH3I studies (J. Phys. Chem. 1997, A101, 5103).