By means of semiempirical extended Huckel calculations, the electronic properties of the surface Pt atoms in the Pt80Fe20(111) alloy have been compared with those of pure Pt(111), Two types of surface Pt atoms exist, depending on the presence or not of a Fe atom as first neighbor in the second layer, An electron transfer occurs from Fe to Pt resulting in an increased electron density on the surface Pt atoms, The transfer is larger for Pt atoms having no Fe as neighbor. All adsorbed molecules have smaller binding energies on the alloy than on pure Pt, This is essentially due to an increase of the repulsions resulting from the increased electron density, An important result is that the adsorption sites and the geometries of the adsorbates are not the same on the alloy and on pure Pt, which explains the modifications in the catalytic properties of Pt when it is alloyed with Fe.