Inverse gas chromatography (IGC) is a versatile tool for the characterization of porous solids. IGC at finite dilution has historically been used for isotherm measurements because of fast equilibration times. A combination of IGC with a flash thermodesorption method allows the separation of micropores and outer surface contributions due to different adsorption mechanisms. Whereas the outer surface and the mesopores have a mono-/multilayer sorption mechanism, the micropores are filled according to the "theory of volume filling of micropores" model. Therefore, a higher energy is required for desorption out of micropores than for desorption out of mesopores and from the surface. This difference is used to calculate two separate isotherms respective to each contribution. Four aluminas with different microporosities are investigated to show the benefit of this approach.