The electric double layer structures on a Pt(111) electrode in H2SO4 solution were studied by in-situ and ex-situ infrared spectroscopies, and the structures were successfully created by a gas phase adsorption of H2O and SO3 under UHV conditions. The absorption bands of hydrogen, neutral water, hydronium cation and bisulfate anion were observed in the in-situ spectra as a function of an electrode potential. Similar spectra could be reproduced by sequential gas doses of H2O and SO3 on Pt(111) at low temperatures under ultrahigh vacuum conditions (ex-situ measurements). The various kinds of electrode reaction were responsible for the real double layer structures as well as the electrode potentials.