Reaction of negatively-charged clusters of carbon dioxide (CO2)N- with CH3I results in the formation of anions with the formulae [(CO2)(n)CH3I](-), [(CO2)(n)CH3](-), and [(CO2)(n)I](-). The product mass spectrum of [(CO2)(n)CH3I](-) displays a distinct bimodal distribution; [(CO2)(n)CH3I](-) with n = 1-3 and n greater than or equal to 7 are observed, while those with n = 4-6 are not detected. Photoelectron spectroscopy of [(CO2)(n)CH3I](-) reveals that [(CO2)CH3I](-) is formed as the core of [(CO2)(n)CH3I](-) with 1 less than or equal to n less than or equal to 3 while CO2- is the core for n greater than or equal to 7. The [(CO2)CH3I](-) core also behaves as chromophore, which absorbs a 532 nm photon to dissociate into CH3CO2- + I or CH3CO2 + I- channels. The chemical identity of [(CO2)CH3I](-) is probed by the combination of photodissociation and photoelectron spectroscopic techniques. Ab initio calculations have also been performed to examine the geometrical and electronic structures of [(CO2)CH3I](-). We conclude from these results that [(CO2)CH3I](-) is composed as a molecular anion of acetyloxy iodide, CH3CO2I-, where the acetyloxyl framework binds the I atom through an O-I bond to share the excess electron. The formation mechanism of CH3CO2I- is discussed on the basis of the observed steric effects on the reaction.