The chemical properties of Pt deposits on Co(0001) in the range of Pt coverage up to 1.5 monolayer (ML) are presented. They are checked by CO adsorption at room temperature. We show that the CO total coverage decreases as the Pt coverage increases although both the (apparently) free Co areas as well as the Pt-covered areas do adsorb CO. This is a consequence of the CO/Pt adsorption energy lowering down to 0.8 eV as evaluated experimentally. This is interpreted in view of the observations performed by STM, LEED and photoemission on the Pt-covered surface, before and after CO exposure. Ab initio calculations performed on a Pt monolayer on Co(0001) confirm the interpretation. (C) 2003 Elsevier B.V. All rights reserved.