The nature of the interactions between the platinum cluster Pt-4 and the adsorbates H-center dot, (OH)-O-center dot, and H2O, as well as the influence of these adsorbates on the electronic structure of the Pt-4 cluster, was investigated by density functional theory (B3LYP, B3PW91, and BP86) together with the effective core potential MWB for the platinum atoms, and 6-311++G(d,p) and aug-cc-pVTZ basis set for the H and O atoms. Identification of the optimal spin multiplicity state and the preferential adsorption sites were also evaluated. Adsorption changes the cluster geometry significantly. but the relaxation effects on the adsorption energy are negligible. The adsorbates bind preferentially atop of the cluster, where high bonding energies were observed for the radical species. Adsorption is followed by a charge transfer from the Pt4 cluster toward radical adsorbates, but this charge transfer occurs in a reversed way when the adsorbate is H,O. In contrast with water, adsorption of the radicals H-center dot and (OH)-O-center dot on platinum causes a remarkable re-distribution of the spin density, characterized by a spin density sharing between the H-center dot and (OH)-O-center dot radicals and the Cluster. The covalent character of the cluster-adsorbate interactions, determined by electron density topological analysis, reveals that the Pt-4-H interaction is completely covalent, Pt-4-OH is partially covalent, and Pt-4-H2O is almost noncovalent.