This paper reports the gas separation properties of two asymmetric hollow fiber membranes prepared from the polyimide Matrimid. One is based on pure Matrimid, the second one is a blend with the polyimide P84. The hollow fiber membranes are used to separate gas mixtures of propylene and propane and in a particular case methane is present as a third component. The C3H6/C3H8-mixed gas selectivity of asymmetric membranes P84/Matrimid-blend is stable in time and decreases with increasing propylene partial pressure from 20 to 12. Propylene causes the propane permeance to increase in binary mixed gas experiments: propylene permeance is however not affected. This indicates plasticization of propane transport by propylene. Plasticization effects do influence the separation performance of asymmetric pure Matrimid membranes as well, but the effect is less pronounced as compared to the blend with P84. The presence of 50 vol.% methane in the feed gas diminishes the effect of C3-plasticization. The comparison with single gas sorption and permeation kinetics leads to a distinction of auto and mutual plasticization.