We have synthesized fluorinated block copolyimides with different block chain lengths by chemical imidization in a two-pot procedure and prepared the asymmetric coployimide membranes using the dry-wet phase inversion process. The gas transport properties of the asymmetric membranes were measured using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 C. We demonstrated that the skin layer thicknesses and the gas transport properties of the asymmetric membranes depended on the copolyimide structures. The phase separation in the block copolyimide solution instantaneously occurred so that the skin layer of the asymmetric block copolyimide membrane became thinner than that of the asymmetric random copolyimide membrane and the gas permeance of the asymmetric block copolyimide membrane had a high value. The apparent skin layer thickness of the asymmetric block coployimide membrane was 230 nm. Polymer membranes are considered an effective technology for the separation of gaseous mixtures due to their high separation efficiency, low operating costs, and simple operation. The development of novel polymer membranes with even higher gas permeabilities and selectivities has received much attention.1,2 The gas permeation through a polymer membrane not only is a function of the chemical structure of the polymers but also is determined by the morphology or the domain structure formed on the membrane. Particularly, in the case of the block copolymers or miscible blends of polymers, their gas permeations are affected by the domain size of microor nanometers formed on the membrane or the extent of the interactions between the component polymers. [3][4][5] Recently, the gas transport properties of block copolymers based on a rubbery polymer have been investigated for acid gas removal from natural gas, recovery of CO 2 from flue gases, and the removal of CO 2 from gas mixtures with hydrogen. 6 However, it is difficult to prepare ultrathin membranes from the block copolymers based on rubbery polymers, because these copolymers do not have sufficient mechanical properties.Polyimide as a glassy polymer has been recognized as one of the most promising potential candidates for a gas separation material because of the high gas selectivity and excellent mechanical properties for preparing the ultrathin membranes. [7][8][9] In general, the polyimide is obtained from a precursor condensed from two monomers with dianhydride and diamine moieties, and the gas transport properties through the polyimide membrane have been investigated. Although the gas transport properties of copolyimide membranes were reported in some papers, 10-12 most of the copolyimides were random copolymers and there has been little attention given to the gas transport properties of block copolyimide membranes.Of course, there are a few papers concerning block copolyimides, which reported the effects of the monomer structure and the length of the block chains on the thermal and chemical stability or mechanical properties of the copol...