The adsorption of water molecules in active carbon in normal condition can be monitored in situ with an oblique-incidence polarization-modulated optical reflectance difference technique. The optical response cannot only characterize the time length of adsorption, but reveal the tendency of dielectric properties of active carbon during the adsorption process. Therefore, the whole adsorption can be described by the permittivity obtained by OIRD measurement. Such a technique is also effective under high ambient pressure and temperature because of the optical detection, indicating that the precise measurement carried with this technique can help in building adsorption theory system in some different conditions.
