Problems concerning analysis of the pore structure of the MCM-41 mesoporous materials were discussed on the basis of nitrogen adsorption. Calculation results from both adsorption isotherms and MID simulations were taken into account in this analysis. It was found that the thickness of an adsorption layer in cylindrical mesopores was higher than that in macropores and it increased with the decrease in the mesopore diameter. This increase was relatively slow for the diameters falling down to ca. 4 nm. For this region. the values of the statistical film thickness can be obtained from the equations (e.g., Harkins-Jura) that are independent of the pore size. For pore diameters lower than 4 nm. the above growth is more pronounced and taking into account the dependence of the adsorption layer thickness on the pore diameter appears necessary. (C) 2002 Elsevier Science B.V. All rights reserved.