In this review membrane gas separation properties of two groups of glassy polymers are considered and compared. The first group comprises highly permeable polymers with large free volume. They contains bulky substituents of the structure MR3, where M = Si, Ge, C, and R = Me, Et, Ph. The second group is represented by polycondensation materials (polyamides, polyimides etc.) with various bulky side groups. It was noted that introduction of such substituents into various main chains of polymers of both groups results in increases in gas permeability, free volume and diffusivity. Robeson diagrams for both cases are considered.