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We demonstrate the synthesis of a microporous covalent-network membrane derived from co-continuous blends of a porogenic urea network and a polyimide (PI). We show that the urea networks in the PI matrix may be thermally rearranged while selectively expelling small molecular fragments, thereby forming a new network bearing reticular microporous molecular pathways. The porous structures enable reverse-selective gas separation, efficiently blocking carbon dioxide to which most polymeric membranes exhibit selective permeability. The proposed method for fabricating microporous organic membranes with highly tunable porosities using a variety of chemical structures and processing parameters is facile and shows promise for the creation of new membrane-based molecular-separation techniques. [ABSTRACT FROM AUTHOR]