The ab initio cluster model approach has been applied to investigate the electric field effects for CO on on-top and bridge sites of the Pt(100) surface modeled by Pt-41 and Pt-32. For the available experimental data in the absence of external electric fields, a reasonable agreement between experimental and calculated results is found for CO chemisorbed on Pt(100). Electric field effects induce changes in the equilibrium distance and vibrational frequency of chemisorbed CO but also on the chemical bonding mechanism, in particular in the extent of the 2 pi* backdonation. The model calculations predict a linear relationship between the intensity of the electric field and the extent of the 2 pi* backdonation. The existence of this linear relationship means that a separation between electrostatic and chemical effects is not feasible.