Properties of the oxygen molecule, atomic oxygen, and intermediate products of its reduction, OH, OOH, H2O2 on (111), (100), and (110) Pt surfaces have been investigated using periodic density functional theory. The Pt surfaces are modeled as four-layer slabs. Adsorption energies and geometries, as well as the charge-transfer properties are calculated. Computed characteristics of the adsorbed oxygen reduction intermediates supply known tendencies of the low index Pt surface activities under different experimental conditions. Electric field dependencies of the properties of all species adsorbed on a Pt-9(111) cluster have been also studied. Lowering the field causes an increase of the O-O bond length of O-2ads, attracting the molecule to the Pt surface and increasing the charge transfer from Pt to 2pi* orbitals of the oxygen molecule. The weakening of the O-O bond is evidenced by a decrease of the O-O stretching frequency. The charge-transfer from the Pt-9 cluster to the adsorbates is observed for all species. In our calculations hydrogen peroxide was unstable on all three low-index Pt surfaces and dissociated into two hydroxyls or a water molecule and atomic oxygen. The results of the calculations are discussed in the context of the mechanism of oxygen reduction. (C) 2004 The Electrochemical Society.