Hydrated carbon dioxide radical anions CO2·-(H2O)n in the gas phase[1] undergo interesting ion-molecule reactions, for instance,with CH3SH to form C-H bond via hydrogen-atom transfer, yielding HCO2-.[2] With olefins, formation of C-C bond has also been demoustrated.[3,4] The mechanism for the reaction of CO2·-(H2O)n with methyl acrylate (CH2=CHCOOCH3) in aqueous media at the molecular level has been examined using Fourier-transform ion cyclotron resonance mass spectrometry and density functional theory.