As a new solvent free method for membrane formation, we have investigated the foaming of high-T-g polymers. We report two different routes for the formation of open-microcellular and open-nanoporous membrane morphologies. Porosity is introduced by expansion of carbon dioxide saturated films and hollow fibers at elevated temperatures. The microcellular structure manifests itself by small spot-like openings with diameters between 50 and 200 nm in the cell walls. The nanoporous structures exhibit a lacy structure with pore sizes between 2 and 50 nm. The mass transport resistance of the membrane structures is characterized in detail on the basis of gas permeation measurements.