NOVELTY - A thin layer composite membrane (c1) incorporates a selective layer comprising a microporous material and a porous supporting layer. The microporous material comprises organic macromolecules containing first generally planar species (s1) connected by rigid linkers having a point of contortion such that two adjacent first planar species connected by the linker are held in non-coplanar orientation.    USE - For modifying the composition of a feed mixture comprising first and second species (preferably alcohol, phenol or a halogenated hydrocarbon compound, metal containing organic compound, oxygen or nitrogen); and separating or enriching oxygen species from a feed mixture (claimed); separation processes of industrial relevance such as separation of hydrogen from hydrocarbons, nitrogen or carbon monoxide in refinery industry; separation of carbon dioxide, water and hydrogen sulfide from natural gas (dewpointing, upgrading); air separation either to obtain enriched oxygen or nitrogen for various applications; separation of volatile organic compounds (VOC) or lower hydrocarbons from air and other gases; pervaporative separation from trace organic compounds found in aqueous streams; separation of low molecular weight compounds and oligomers from fluids especially organic solvents (nanofiltration).    ADVANTAGE - The membrane systems exhibit high selectivity and high permeability. The performance of composite membranes is more reliable, predictable and controllable than known membranes. The high intrinsic microporosity of the microporous materials incorporated into composite membranes also provides the opportunity to produce membranes exhibiting significantly improved performance (gas permeability and selectivity) than conventional membranes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a method (M1) for producing (c1) involving dissolving the microporous material in a solvent to form a solution of the microporous material; contacting the supporting layer with the solution; and evaporating the solvent to provide the selective layer comprising the microporous material on the supporting layer;    (2) a method (M2) for modifying the composition of a feed mixture comprising first and second species involving applying the feed mixture to a feed side of (c1), collecting a retentate possessing a different composition to the composition of the feed mixture from the feed side of the membrane and/or collecting a permeate possessing a different composition to the composition of the feed mixture from an opposite side of the membrane;    (3) a method (M3) for separating a first species from a mixture of the first and second species involving applying the mixture to a feed side of (c1), causing the first species to pass through (c1), and collecting the first species from the opposite side of the membrane;    (4) a method (M4) for enriching a first species in a first mixture of containing first and second species involving applying the first mixture to a feed side of (c1), causing the first mixture to pass through (c1), and collecting a second mixture of the first and second species, that is enriched in respect of the first species compared to the first mixture, from an opposite side of the membrane and/or collecting a third mixture of the first and second species, that is enriched in respect of the second species compared to the first mixture, from the feed side of the membrane.