As a new method of membrane formation, we have investigated microcellular foaming of thin (similar to 100 mum) polysulfone films containing varying trace concentrations of tetrahydrofuran using carbon dioxide as a physical blowing agent. Membrane morphologies were obtained by first saturating the polymer with carbon dioxide at 5 MPa, and subsequently heating the sample above the glass transition temperature (T-g) Of the polymer/gas mixture at atmospheric pressure. The presence of tetrahydrofuran in the polymer at concentrations above 0.04 wt.% led to a transition from a closed cellular structure into novel open-cellular morphologies. The open structure manifests itself by small spot-like openings (diameters between 10 and 100 nm) in the cell walls. The mass transport resistances of the porous films were quantified using gas permeation measurements, and a Knudsen-type separation mechanism was observed. Detailed investigation showed that the transport resistance can mainly be controlled by two variables: (1) the concentration of the residual solvent in the polymer film, and (2) the foaming temperature. At optimal foaming temperatures, thin cell walls are obtained, which break up when fluctuations in the wall thickness are amplified by plasticizing solvent molecules.