The chemical reactivity of cobalt cluster anions, Co n -(n ) 2-8), toward O 2 , N 2 , and CO is investigated by flow tube reactor kinetics. The reactivity is highest for the reactions with O 2 and intermediate for CO, while the clusters are nearly inert toward N 2 . Only the largest clusters studied, Co 7 -and Co 8 -, show any evidence for reaction with nitrogen by addition of N 2 at very slow rates. Reactions with O 2 have rapid rate coefficients that are largely independent of cluster size and lead to extensive fragmentation of the clusters. The rate coefficients with CO increase with increasing cluster size. Sequential addition of CO to the cobalt cluster anions leads to the saturated species: Co 2 (CO) 7 -, Co 3 (CO) 10 -, Co 4 (CO) 12 -, Co 5 (CO) 13 -, and Co 6 (CO) 15 -. Skeletal structures are proposed for these saturated clusters in accordance with electron-counting rules.