MCM-41 siliceous molecular sieves were used to test the applicability of the Kelvin equation for nitrogen adsorption in cylindrical pores of the size from 2 to 6.5 nm. It was shown that the Kelvin equation for the hemispherical meniscus, corrected for the statistical film thickness, is in quite good agreement with an experimental relation between the pore size and the capillary condensation pressure. The agreement can be made quantitative in the pore size range from ca. 2 to 6.5 nm, if a simple correction to the Kelvin equation is introduced. The required statistical film thickness curve (t-curve) was calculated using nitrogen adsorption data for large pore MCM-41 samples and the obtained results were extrapolated using an adsorption isotherm for a macroporous silica gel. Moreover, an accurate analytical representation of the t-curve was found. Since both the corrected Kelvin equation for cylindrical pores and the t-curve have simple analytical forms, they can conveniently be used in a variety of methods to evaluate porosity. It was shown that the BJH method with the corrected Kelvin equation accurately reproduces pore sizes of MCM-41 materials. A comparison was made between the specific surface areas for the MCM-41 samples calculated on the basis of the BET equation and those obtained using other independent methods. The results strongly suggest that when nitrogen adsorption data are used, the BET method overestimates the specific surface area of siliceous materials. The latter conclusion was supported by the examination of the obtained statistical film thickness curve.