Nitrogen adsorption isotherm,,, at 77 K and high pressure adsorption of supercritical methane at 303 K were measured on mesoporous carbon (CMK-1) prepared from the templating of MCM-48. These experimental isotherms were simulated with grand canonical Monte Carlo (GCMC) simulation using the rod-aligned slitlike pore (RSP) model. This RSP model gives good agreement with the experimental nitrogen adsorption isotherm in the low-pressure region, while it describes well the observed methane isotherm. The isothermal compressibility of the adsorbed methane at 303 K was calculated from the experimental and simulated isotherms. The isothermal compressibility of the adsorbed methane shows the intermediate state between the supercritical and liquid phase. This indicates that the adsorbed layer of supercritical methane is highly compressed by the strong interaction potential from the pore walls.