Density functional theory calculations are used to characterize the influence of the nanoparticle-substrate interaction on CO adsorption in various Pt cluster sizes. Adsorption energies are evaluated on different sites of clusters supported on graphite and on unsupported clusters. Charge transfer and density of states are examined in order to determine the influence of the support on the cluster electronic properties. In comparison with adsorption on substrate-free cluster, we find weaker CO adsorption energy when CO is adsorbed on top of the cluster and stronger adsorption energy when CO is adsorbed on sites near the surface. The substrate has a larger influence on CO adsorption in sub-nanometer clusters; however some effect is detected in a 1 nm-diameter cluster. The graphite support has the capacity to accept electrons from the deposited cluster, but can donate electrons when the cluster adsorbs CO. The density of states of the CO molecule when it is adsorbed on clusters with different carbon substrates shows particular features that reveal the capacity of the support to influence CO adsorption. (C) 2013 Elsevier B.V. All rights reserved.