The selection of a polymer for use as a gas separation membrane involves consideration of a number of intrinsic qualities that dictate many of the performance characteristics of the separation system. Gas permeation properties are the most obvious and are certainly critical, but other attributes such as solubility, mechanical behavior, cost, etc. must be considered as well. Severe deficiencies in some of the latter areas can preclude a polymer with attractive permeation characteristics from commercial use as a membrane. Nevertheless polymers that do not have outstanding permselective properties will not even be considered for the next generation of membrane materials. 1–5