In our recent brief communicaton, we have proposed a new method for the evaluation of the pore entrance size in silicas and other materials with cage-like mesopores. This method is based on the modification of the material with ligands that are capable of forming a dense monolayer chemically bonded on the surface. The size of the smallest ligand that is capable of rendering the cage-like pores inaccessible for gas molecules is related to the pore entrance size. In the case of silicas, this method allows one to readily evaluate pore entrance diameters between about 1 and 5 nm. Herein, details of this novel approach are reported. It is shown that the introduction of the surface ligands can be monitored using thermogravimetric analysis under air, whereas the elemental analysis does not allow one to differentiate between carbon contents from chemically bonded ligands and trapped organic species. The surface modification inherently leads to the adsorption capacity loss, which depends on the pore diameter, amount of framework micropores, and surface modifier loading. It is suggested how to distinguish the adsorption capacity loss related to the pore blocking from that merely resulting from the surface groups introduction. Differences in the pore blocking behavior for cage-like pores and channel-like pores are demonstrated. Unprecedented, one-step micropore filling with nitrogen at relatively high pressures is reported for a material with narrow cylindrical pores with a hydrophobic surface. This work provides a solid foundation for the use of the surface modification method in the elucidation of the pore entrance size.