Using a cluster model, we investigated the similarities and differences in chemical activity and the magnetic properties of Sc-n clusters (n = 2-13) and their oxides, ScnO, toward CO molecule adsorption via a spin polarized density functional theory approach. The Sc-n and ScnO clusters have similar chemical activity at small sizes of n = 2-10, whereas remarkable differences are observed at large sizes of n = 11-13. More interestingly, different magnetic responses are found in the Sc-n and ScnO clusters with the presence of CO molecule: The magnetic moment is attenuated significantly for Sc-n with n = 2, 4, 12, and 13, whereas for ScnO, it is enhanced at n = 4 and 13 and is reduced for n = 7, 8, 10, and 11. In particular, the magnetic moment remarkably increases from 7 mu(B) of Sc13O to 13 mu(B) of Sc13OCO, whereas it reduces from 19 mu(B) of Sc-13 to 5 mu(B) of Sc13CO.