The present paper discusses the tendency of solution-cast Hyflon (R) AD membranes to retain unexpectedly high amounts of solvent, the possible reasons of this phenomenon and its effect on the membrane performance. Dense membranes, prepared by solution-casting and subsequent evaporation, showed large differences in their thermal, mechanical and transport properties, depending on the residual solvent content. Complete solvent removal required heating under vacuum up to well above the glass transition temperature. Analysis of the permeability, diffusion and solubility coefficients of six permanent gases showed that plasticization by the residual solvent reduces the permselectivity and increases the permeability. Data of solution-cast membranes after complete solvent removal compare well with those of a melt-pressed sample. Experimental gas transport parameters were confronted with simulated data, obtained by the Gusev-Suter Transition-State Theory (TST) method and by molecular dynamics (MD) simulations. H-1 High Resolution Magic Angle Spinning Nuclear Magnetic Resonance spectroscopic analysis of the residual solvent in the polymer matrix did not reveal a particular interaction between polymer and solvent, suggesting that the solvent retention is mainly diffusion controlled. (c) 2007 Elsevier Ltd. All rights reserved.