The electric field dependence of CO and NO adsorption onto a Pt(lll) cluster model surface is studied by quantum-chemical density functional calculations. It is found that CO starts favoring the hollow site over the atop site for negative potentials, whereas NO adsorbs hollow sites, at ail potentials. These differences can be understood by referring to the Blyholder 5 sigma donation - 2n* back donation model, to the influence of the field in shifting the adsorbate levels with respect to the metal levels, and to the lower energy of the NO 5 sigma and 2 pi* adsorbate levels with respect to CO. It is also found that the interfacial Stark tuning rate (i.e. the change in intramolecular stretching frequency with the field) is higher for NO and higher in the high-coordination site, which can be understood in terms of the back donation influence. These findings are in good agreement with experimental results.