The coadsorption of H2O and SO3 on Pt(111) has been studied as a model system for electrolyte-interface interaction under UHV conditions. The model double layer structures were characterized by low energy electron diffraction and infrared spectroscopy of water, bisulfate and sulfuric acid molecules adsorbed on a model surface at temperatures ranging between 110 and 400 K. They were compared with an actual double layer structure on a M(111) (M = Pt, Ph, Au, Cu and Ag) electrode surfaces in a 0.5 M H2SO4 acid solution under active potentials. Bisulfate ions (400-750 mV) and sulfuric acid molecules (750-1200 mV) adsorbed on a Pt(111) electrode under an active electrode surface were reproduced by a stepwise dehydration from the coadsorbates of H3O+ and HSO4- at 190 and 260 K, respectively. The model structure of the former bisulfate phase was (root 3x root 7), while that of the latter neutral sulfuric acid molecule phase was (root 3x root 3) R30 degrees. The former bisulfate was found on all the surfaces, but the latter neutral sulfuric acid molecule was hardly observed on the Au, Cu and Ag surfaces.