A new method to determine the absolute amount of adsorption in the case of high-pressure adsorption of supercritical gases was proposed. We obtained the volume of the physisorbed layer by iteration procedures using the mean density of the adsorbed phase which was determined with the aid of the surface excess mass isotherm on nonporous materials. This new approach was applied to high-pressure CH4 adsorption on nonporous carbon black and microporous carbon at different temperatures from 273 to 303 K, providing the thickness of the interfacial layer of physisorbed molecules from the volume of the physisorbed layer and the external surface area. The interfacial layer thickness of physisorbed molecules under the high-pressure gas was in the range of 1.9 to 29 nm.