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(alpha) (SCC-DV-X(alpha)) 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 C=O 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.