The density functional theory has been applied to study the interaction of platinum (atoms, dimers, and layers with equal number of platinum atoms as oxygen surface atoms) with the (100) surfaces of MgO and BaO. The systems are characterized with respect to preferred geometrical configurations and electronic structure. Platinum is found to interact considerably stronger (similar to1.5 eV) with BaO than with MgO. The mapping of the potential energy surface suggests that monomer diffusion at room temperature is unlikely on both studied supports. Dimer diffusion, however, should be possible to observe at room temperature on MgO(100). For layer adsorption, a marked difference is predicted between MgO and BaO. Whereas the Pt/MgO interaction is weak, the Pt/BaO interaction is close to that of atoms and dimers. Substrate induced modifications of chemical properties are also investigated by studying variations in CO adsorption energy. CO is predicted to bind more strongly to supported atoms and dimers than to the corresponding gas phase clusters or the Pt(111) surface. Moreover, adsorbed CO is found to enhance the platinum/support bond strength. (C) 2003 American Institute of Physics.