The reactivity of Co deposited on Cu(lll) is determined using the dissociation of CH4 as a test reaction. The CH4 molecules are produced in a supersonic molecular beam expansion and dissociate on Co but not on Cu under the experimental conditions used here. It is found that the dissociation probability of CH4 on a few layers of pseudomorphic Co adsorbed on noble Cu(lll) is larger than that of pure Co. The apparent CO binding energy is, however, not shifted as measured using thermal desorption. A model of this surprising behavior is presented using information on the structure and theoretical calculations on the shift of the d-band of Co adsorbed on Cu(lll).