Plasticization is a phenomenon frequently encountered in the application of glassy polymeric materials for solution-diffusion membranes. Conventional methods for stabilizing the membrane are either annealing or cross-linking, which hardly influence the selectivity of the membrane, but decrease the permeability. For single-gas experiments, the literature shows that plasticization can be stabilized by blending a polymer with high plasticization tendencies with one that is hardly affected by the sorbed molecules. Most permeation experiments are carried out with pure CO2, but little is known about the transport properties determined from mixed gas experiments. Stabilization of plasticization in mixed-gas experiments for polymer blends of the polyimide Matrimid and polysulfone is reported, as well as transport properties of a new homogeneous polymer blend based on Matrimid and the copolyimide P84. Experimental results show that the material is stabilized against carbon dioxide plasticization and selectivity for a carbon dioxide/methane mixture significantly improves.