The acetate radical anion, CH2CO2.-, has been generated in the gas phase at room temperature and its thermochemical properties and reactivity have been examined with use of a flowing afterglow-triple quadrupole instrument. This ion is formed in high yield from the reaction between F-2 and the enolate ions of either acetic acid or trimethylsilyl acetate. Collision-induced dissociation (CID) of CH2CO2.- occurs by loss of CO2, forming CH2.- with a measured threshold energy of 60.9 +/- 2.7 kcal/mol. The (oxygen) proton affinity of CH2CO2.- (Delta H-acid-[(CH2CO2)-C-.-H]) has been determined to be 347.0 +/- 1.1 kcal/mol from measurements of the relative yields of the carboxylate ion fragments resulting from CID of proton-bound dimer ions formed by termolecular association of CH2CO2.- with carboxylic acids with known gas-phase acidities (i.e., by the Cooks kinetic method). This result indicates that removal of a hydrogen atom from the alpha-carbon of acetic acid (Delta H-acid(CH3CO2H) = 348.6 +/- 2.9 kcal/mol) increases the acidity by 1.6 kcal/mol. These data are used to derive the 298 K heat of formation for acetate radical anion, Delta H-f,H-298(CH2CO2.-) = -78.2 +/- 2.7 kcal/mol, and the C-H bond dissociation energies D-298-[(O2CCH2)-O---H] = 93.7 +/- 4.0 kcal/mol and D-298[HO2CCH2-H] = 95.3 +/- 2.9 kcal/mol. The acetate radical anion undergoes gas-phase reactions with NO, SO2, and NO2 by CH2.- transfer, forming CH2NO-, CH2SO2.-, and CH2NO2-, respectively, and reacts with CH3SSCH3 by CH3S abstraction. Hydrogen atom transfer is shown to occur during the formation of cluster ions of CH2CO2.- with certain carboxylic acids.