NOVELTY - A gas separation membrane comprises a porous substrate (10) having a first surface region (10a) and second surface region (10b)between which the mixture of gas species flows, a sealing polymer layer (20) of different composition to the porous substrate that forms a continuous coating across the second surface region of the substrate and permeable to the mixture of gas species, a selective polymer layer (30) in the form of a cross-linked macromolecular film, where the selective polymer layer is located on and covalently coupled to the sealing polymer layer.    USE - Gas separation membrane for separation of a target gas species such as carbon dioxide from a mixture of gas species, and industrial processes such as production of oxygen enriched air, separation of moisture or carbon dioxide from natural gas, and recovery or capture of a desired gas species form vented gases such as flue gas from coal and natural gas power stations.    ADVANTAGE - The membrane has better thermal mechanical properties, better structural stability, improved permeability, improved selectivity and reduced manufacturing cost.    DETAILED DESCRIPTION - A gas separation membrane comprises a porous substrate having a first and second surface region between which the mixture of gas species flows, a sealing polymer layer of different composition to the porous substrate that forms a continuous coating across the second surface region of the substrate and permeable to the mixture of gas species, a selective polymer layer in the form of a cross-linked macromolecular film, where the selective polymer layer is located on and covalently coupled to the sealing polymer layer, and has a high permeability to the target gas species relative to other gas species present in the mixture of gas species that is to be subjected to separation. INDEPENDENT CLAIMS are also included for the following:    (1) a method for preparing a gas separation membrane    (2) a method for separating a target gas species from a mixture of gas species.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a gas separation membrane.    Porous substrate (10)    first surface region (10a)    Second surface region (10b)    sealing polymer layer (20)    Selective polymer layer (30)