The different adsorption processes and conformational structure of water molecules at a 0.1 M HClO4/Pt(110) interface have been studied by using in-situ infrared-visible sum-frequency generation (SFG) spectroscopy. The O-H stretch resonance is found to be quite weak, while the Pt-H stretch mode arises rapidly in the hydrogen adsorption region and the total symmetric stretch mode nu(1) of ClO4- anions dominates the resonant signal in the double layer region. In this work we provide the first spectroscopic evidence that hydrogens and ClO4- anions are more favored than water molecules, despite their significant dipole moment, in the competitive adsorption processes below the oxidation potential. Our SFG results suggest that, for a platinum in the 0.1 M HClO4 solution, most interfacial water molecules are not directly adsorbed but rather are present at the electrode surface as the constituents of hydration shells. (C) 2003 American Institute of Physics.