We report gas-phase reactions of free Co-n(CO)(m)(+) (n = 3-11, m = 0-2) with H-2, expecting a catalytic reaction of coadsorbed CO and H-2 on Co-n(+). Preadsorption of CO molecules is found to promote H-2 adsorption, in particular, on Co-n(CO)(+) (n = 5, 8-10). Density functional theory (DFT) calculations reveal that the reactivity is governed by the molecular-orbital energy of Co-n(+), which is tuned by preadsorbed CO molecules. Collision-induced-dissociation experiments performed on ConCOH2+ (n = 8-10) imply that at least some of the CO and H-2 molecules are bound together on Co-n(+).