The mass transport of asymmetric membranes for the separation of carbon dioxide/methane mixtures is determined by competitive sorption and plasticization. With increasing feed pressure in mixed gas experiments, the selectivity decreases due to both effects. Distinction whether one or the other mechanism is responsible for the selectivity loss is important since competitive sorption is related to intrinsic material properties and cannot be tailored, whereas plasticization can be suppressed by various chemical means. This paper describes the systematic analysis for five different asymmetric membranes with respect to the balance between competitive sorption and plasticization. Four asymmetric membranes where prepared for this study, one membrane was based on a commercial precursor. Of these membranes, three are based on the polyimide Matrimid: pure Matrimid, and blends of Matrimid with polyethersulfone as well as Matrimid with a polyimide P84. These membranes are compared with two other ones: cellulose acetate and polyphenyleneoxide PPO. The blend of Matrimid with P84 shows the highest mixed gas selectivity and is very resistant against plasticization without any further chemical modification.