A new determination method of an absolute adsorbed amount n(ab) from the surface excess mass n(ex) for high-pressure adsorption isotherms of a supercritical gas was proposed. The effectiveness of new method was examined by using the density-functional theory (DFT). The DFT study showed that this analysis can provide reasonable results; both of the absolute adsorbed amount determined from the proposed method and the DFT agreed within 5% at 90 MPa and only 1% below 5 MPa. Furthermore, we applied this new method to the experimental surface excess isotherm in the literature, which has a maximum. The analyzed absolute adsorption isotherm from the surface excess adsorption isotherm having a maximum were of IUPAC type I or type II. This method gave the thickness of the interfacial layer of the adsorbed phase. (C) 2001 American Institute of Physics.