Permeation experiments with He, N-2, O-2 and CO2, have been carried out with double layer composite membranes consisting of a silicone rubber support layer and a thin polyimide layer determining the permeation properties, The estimated thickness of the polyimide layer in the composite membranes was between 1.5 and 4 microns. This paper describes the phenomenon of accelerated plasticization of such thin polyimide layers used in gas separation membranes. The pressure-normalized fluxes were determined at 5 bars for N-2, O-2 and in the range of 1-8 bars in the case of He and CO2. Helium permeation decreased with increasing feed pressure and no hysteresis behavior was found for successive increasing and decreasing feed pressure steps. For CO2 the pressure-normalized flux did not follow the typical behavior of glassy polymers but increased continuously with increasing feed pressure. Also, CO2 permeation showed a clear hysteresis effects resulting in an increasing permeability with time and an elevated magnitude in successive decreasing feed pressure steps. Hence. stronger plasticization effects in the composite membranes must be concluded in comparison with thick single film membranes. The plasticization effects are significantly pronounced as the polyimide concentration in the solution from which the membranes are cast become more dilute. The latter is interpreted as accelerated plasticization with decreasing film thickness.