The Database "Gas Separation Properties of Glassy Polymers," which was developed at the Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences in the late 1990s and updated at regular intervals, is presented in this paper. This database is currently not only an electronic handbook but also a useful tool to search for novel membrane materials with improved properties for solving traditional and new gas-separation problems and for testing various models using a large body of data. To date, the database was also appended with information on the free volumes of polymers measured by positron annihilation lifetime spectroscopy. Permeability coefficients can be calculated for novel polymers by means of a group contribution method using this database. The paper highlights some features of group contribution methods with specific examples.