We have developed a new type of asymmetric membrane that has a hyper-thin skin layer and sponge-like porous matrix. 6FDA-BAAF polyimide was used as one of the membrane materials. The skin layer had hardly any defects and the thickness of the membrane was 40-60 nm. The skin layer and sponge-like porous matrix, which had a finger-void-free morphology, were formed as one body. The interface between them was found to be clearer than with other asymmetric membranes and the skin layer did not form as a graded density skin. It is suggested that the formation of such hyper-thin skin layers depends on the dissolution property between a dope solvent and water as a solidification solvent. Namely, solvents such as diethylene glycol dimethyl ether (DGDE; used in this study) are miscible in water, but a sharp interface between DGDE and water forms for a while like between water and oil. The hyper-thin skin layer having hardly any defects is possibly formed by solidification at the sharp interface. It is obvious that such solvent properties might depend on the dielectric constant and/or dipole moment. The selectivity and permeation rate of the 6FDA-BAAF polyimide asymmetric membrane had an alpha of 27 for CO2/CH4 and a permeation rate of 1.3 [Nm(3)/m(2) h atm] (= 5.5 x 10(-4) [cm(3)(STP)/cm(2) s cmHg]) for CO2. (The maximum value had an alpha of 35 for CO2/CH4 and a permeation rate of 1.6 [Nm(3)/m(2) h atm] (= 5.8 x 10(-4) [cm(3)(STP)/cm(2) s cmHg])).