Adsorption of NO molecules on gas phase cobalt cluster ions, Co-n(+) (n = 4-9), was investigated in thermal equilibrium with He gas at 300 K. The Co-n(+) clusters, contrary to the isolated clusters in a vacuum, adsorbed NO without undergoing significant dissociation. Thermal desorption spectroscopy of Co-n(+)(NO)(m) indicated that Co-n(+) clusters with n = 4-6 and n = 7-9 can have four and six adatoms chemisorbed, respectively. Reduction of NO occurred, releasing N-2 molecules, to form Co(n)(+)Ok(NO)(m-k) (k = 2, 4, ...). The reaction mechanism involved the exchange of chemisorbed N atoms with the O atom in NO bound to the clusters. The reactivity of Co-n(+) (n = 4-9) exhibited periodic n dependence, and Co-6(+) and Co-9(+) was similar to the case of the isolated Co-16(+) clusters holding up to eight adatoms reported by Anderson et al. (J Chem. Phys. 2009, 130, 10992-11000).