Electron transfer from a seeded supersonic beam of alkali-metal atoms to a crossed molecular beam of CO2 clusters was found to form predominantly (CO2)n- anions, with n = 2-16 under our experimental conditions. The mass spectrum has especially prominent anion peaks at "magic numbers" of n = 4, 7, and 14. Threshold energies for formation of the anion clusters with n = 2-10 were determined by measuring the yield as a function of collision energy. The nominal electron affinities corresponding to these threshold energies are consistent with an impulsive model which postulates that the electron attachment occurs to a subunit within the clusters. This implies that cluster anions larger than the subunit moiety are produced with substantial vibrational excitation. As interpreted with the impulsive model, the threshold data give an approximate value (uncertain by about ±0.2 eV) for the adiabatic electron affinity of the subunit moiety, but do not unequivocally identify its size. Thus we find EA = 0.8 eV if the electron attachment occurs to a dimer and EA = 0.1 eV if to a trimer.