There is provided a composite hollow fiber membrane for gas and vapour separation comprising: a porous membrane substrate; and a selective layer of cross-linked polydimethylsiloxane (PDMS) provided on a surface of the porous membrane substrate, wherein the molecular weight of the cross-linked PDMS is ≥100 kg/mol. There is also provided a method of forming the composite hollow fiber membrane, and a method of forming the cross-linked polydimethylsiloxane (PDMS) having a molecular weight ≥100 kg/mol.