The reaction of cobalt cluster anions Co_n^– (3 ≤ n ≤ 17) with CO₂ was studied experimentally and theoretically to explore the size-specific activation mode of CO₂ by Co_n^–. Mass spectrometric measurements revealed that the reactivity depends strongly on cluster size: the reactivity emerges abruptly at n = 7, peaks at n = 8–10, and then gradually decreases with increasing n. Infrared multiple photon dissociation spectra of Co_nCO₂^– exhibit a single peak at ∼1870 cm^–1, similarly to the previously reported spectra of Co_nCO^–. Density functional theory calculations for Co₇CO₂^– as an example revealed that the dissociative adsorption of CO₂ into CO and O is energetically more favorable than nondissociative adsorption. The infrared spectra calculated for dissociated isomers Co₇(CO)O^– reproduced the experimental results, whereas those for nondissociated isomers Co₇CO₂^– did not. The photoelectron spectra of Co_nCO₂^– were shifted dramatically toward higher energies relative to those of Co_n^–, suggesting electron transfer from Co_n^– to the CO and O ligands. These results indicate that the CO₂ molecule adsorbs dissociatively on Co_n^–, in sharp contrast to its nondissociative adsorption onto the Co monomer anion