In situ IRAS at a Hg-Au electrode in HClO4 Solutions shows that the adsorption state of water molecules on mercury changes depending on the applied potential as in the case on gold rather than on platinum. At the negatively charged electrode surface, it is expected that the dipole moment of the adsorbed water molecules is oriented toward the surface to some extent. The mercury-water interaction is considered to be stronger than the gold-water interaction. As the sample potential changes in the positive direction, the adsorbed water molecules are expelled and/or reorient and perchlorate ions adsorb on the Hg-Au electrode surface.