The electric double layer at electrolyte-electrode interface has been investigated by an experimental simulation under ultra high vacuum (UHV). In order to reproduce an electric double layer on an electrode surface, water is adsorbed on the surface treated with electrolyte components (HCl, Na and sulfuric acid) and CO. The coadsorption of water molecules with the electronegative and electropositive additives indicates the existence of various hydration structures or orientation of water molecules on the surfaces. By comparing the results under UHV conditions with those from in-situ electrode surfaces in solution, there appears to be a correlation between orientation of water molecules and electrode potential. Over-layer water molecules as well as adsorbed ones on the surface provides ordered or disordered structures that determine the electrode potential values.