The adsorption of CO on a Pt electrode was used as a molecular probe, and the surface properties with respect to the electronic structure of the Pt electrocatalyst during modification by S or Bi adatoms were investigated by a combination of cyclic voltammetry, in-situ Fourier transform IR (FTIR) spectroscopy and cluster model analysis using self-consistent charge discrete variational X α (SCC-DV-X α) calculations. It is found that the S adatom decreases the ability of the Pt surface to donate electrons, resulting in weakened CO adsorption and increasing vibrational frequency of the CO band of adsorbed CO molecules. However, the opposite effects were observed in the case of Bi adatoms. The electronic and vibrational properties of the potential dependence of CO adsorbed on a Pt electrode were also studied carefully. The results of quantum chemistry calculations are in good agreement with the experimental results of in-situ FTIR spectroscopy and also reveal the deeper significance of the IR spectroscopy data.