The CO-Pt(110) in nonaqueous electrolyte electrochemical interface was studied by infrared reflection-absorption spectroscopy (IRAS) and infrared (IR)-visible sum-frequency generation (SFG) nonlinear vibrational spectroscopy over a wide range of applied potentials (-1.7less than or equal toPhiless than or equal to1 V/NHE). The integrated intensities of the IRAS and SFG peaks associated with resonant excitation of the atop CO internal stretch vibration (A(IR) and A(SFG)) showed distinct variations with Phi. The influence of vibrational and electronic properties on the observed variations is discussed. Potential dependent dynamical charge transfer is not sufficient to explain the observed value of partial derivativeA(IR)/partial derivativePhi. It is shown that screening factors due to dipolar interactions between molecules within the compressed adsorbed layer must be taken into account in order to explain the variations of the IRAS and SFG data and that the observed behavior differences of A(IR) and A(SFG) with the potential are not necessarily related to changes in the Raman cross section of the adsorbate. (C) 2003 American Institute of Physics.