DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm-1/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm-1/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm-1/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm-1/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations.