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, Rh, 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 3 x root 7), while that of the latter neutral sulfuric acid molecule phase was (root 5 x 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.