We investigated the reactivity of CeO2-supported Pt-4 cluster (denoted as Pt-4/CeO2(1 1 1)) toward O-2 adsorption and dissociation as well as the geometry/electronic properties associated with such metal oxide supported cluster system using density functional theory and on-site Coulomb interaction correction via the Hubbard-like term, U(DFT+U). It was found that Pt-4 binds strongly to CeO2(1 1 1) via Pt-O-Ce bonds which act as "anchors" between the surface and the cluster, confirming its non-sintering as found in experiments. The adsorption of the cluster involves net electron transfer to CeO2, however, charge redistribution also happens within the cluster (from Pt atom bonded to the surface to the Pt on top of the cluster). This charge couples to the top Pt leading to reduce its spin moment as compared to that of unsupported cluster. When O-2 adsorbs on Pt-4/CeO2(1 1 1), while it prefers Pt vertex site near the CeO2 surface, the O-O bond elongation is more profound at the Pt-Pt edges. The energy barrier for dissociating O-2 from this edge site precursor state is smallest. A correlation between the O-O bond length at the precursor state and the stability at the transition state is revealed. Finally, the barrier for dissociation in unsupported Pt-4 is lower, indicating suppression of the cluster's reactivity due to the support. We attribute this to the hybridization of Pt-5d orbitals with O-2p orbitals in CeO2(1 1 1) leading to the broadening of Pt-5d states near the Fermi level. (C) 2013 Elsevier B.V. All rights reserved.