The microporosity of zeolite H-mordenite has been modified by incorporation with silicon modifying groups derived from chemisorption of disilane. The modifying group, occupying more than 110 Angstrom(3) in the main channel, progressively reduces the pore size of the zeolite with precision that can be less than 1 Angstrom. This is indicated by continuous exclusion of gradually smaller molecules (p-xylene, propane, ethane, and methane) from the sorption. As the modification degree is further increased, the side pockets of H-mordenite become blocked and are inaccessible to nitrogen and oxygen. Substantial changes in sorption selectivities of oxygen versus nitrogen have also been observed on the modified zeolite after the protons in the main channels have been replaced by the silicon entities. A study of the thermodynamic properties and the contributions of sorption energy has revealed, for the first time, that such changes in molecular selectivities are directly related to the interplay of the pore size and the polarity of the substrate controlled by the modification process.