High-resolution electron energy loss spectroscopy (HREELS) has been used to study vibrations of CO molecules chemisorbed at low coverage on a Pt25Ni75 sample after different surface treatments. In agreement with earlier work using other methods, we always observe that Pt is enriched in the outermost layer, with a chemical composition strongly dependent on the annealing temperature. We discuss to which extent HREELS of CO may be exploited to "titrate" the surface composition of binary alloys.