Using angle-resolved photoemission spectroscopy, we show direct evidence for charge transfer between adsorbed molecules and metal substrates, i.e., chemisorption of CO on Pt(111) and Pt–Sn/Pt(111) 2 × 2 surfaces. The observed band structures show a unique signature of charge transfer as CO atoms are adsorbed, revealing the roles of specific orbital characters participating in the chemisorption process. As the coverage of CO increases, the degree of charge transfer between CO and Pt shows a clear difference to that of Pt–Sn. With comparison to density functional theory calculation results, the observed distinct features in the band structure are interpreted as back-donation bonding states formed between the Pt molecular orbital and the 2π orbital of CO. Furthermore, the change in the surface charge concentration, measured from the Fermi surface area, shows that the Pt surface has a larger charge concentration change than the Pt–Sn surface upon CO adsorption. The differences between Pt and Pt–Sn surfaces are due to the effect of Pt–Sn intermetallic bonding on the interaction of CO with the surface.