The geometries and binding energies of CO 2 adsorbates on Pt(100) and Pt(111) cluster surfaces were calculated by means of an improved version of the extended Hiickel molecular orbital method. The polarization of the surface by an applied electric potential and coadsorption of H atoms were included in the model. For simulated applied potentials in the range -1.0 to 1.0 V, CO 2 coordination geometries (side-on, formate) involving two adsorbate atoms bonded to the surface are favored, regardless the surface topology and the presence ofcoadsorbed H atoms. In agreement with experiment, larger binding energies are always calculated for the Pt(100) cluster surface.