This paper presents the examination of 14 microporous active carbons. The Dubinin-Astakhov equation for adsorption processes in micropores modified by a function F(z) for gamma distributed Bore sizes was used for obtaining the micropore size distribution based on experimental data of benzene adsorption isotherms. This distribution can be used to determine the fractal cut-off values chi(min) and chi(max). It is shown by means of fractal geometry how to get the structural parameters porosity epsilon and tortuosity tau. Both values are needed for the further calculation of diffusion coefficients according to the Knudsen diffusion equation. A comparison with experimental data shows a very good agreement between calculated and measured values.