An interfacial polymerisation process for forming a thin film composite membrane for gas separation or liquid filtration, the process comprising the steps of: (a) impregnating a porous support membrane with a first reactive monomer solution comprising: (i) a first solvent for the said first reactive monomer; (ii) a first reactive monomer; (b) contacting the impregnated support membrane with a second reactive monomer solution comprising: (i) a second solvent for the second reactive monomer; (ii) a second reactive monomer; wherein: the first solvent and the second solvent form a two-phase system; at least one of the first and second reactive monomers containing concavity, where concavity-containing monomers are molecules comprising at least one spiro-contorted centre, at least one bridged ring moiety and/or sterically congested single covalent bonds around which there is restricted rotation; the reaction of the first and second reactive monomers results in the in-situ formation of a covalently cross-linked 3-dimensional polymeric network possessing intrinsic microporosity, said intrinsic microporosity being defined as a continuous network of interconnected intermolecular voids, said voids arising from the structure of the first and second monomers; and the reaction of the first and second reactive monomers results in a separating layer forming on the support membrane to give a composite membrane; (c) optionally, after a reaction period, capping the unreacted groups of the separating layer with functional groups to modify the surface chemistry; (d) after a reaction period, immersing the resulting composite membrane into a quench medium; and (e) optionally, curing the membrane with temperature or microwaves for a given time. (a) impregnating a porous support membrane with a first reactive monomer solution comprising: (i) a first solvent for the said first reactive monomer; (ii) a first reactive monomer; (i) a first solvent for the said first reactive monomer; (ii) a first reactive monomer; (b) contacting the impregnated support membrane with a second reactive monomer solution comprising: (i) a second solvent for the second reactive monomer; (ii) a second reactive monomer; wherein: the first solvent and the second solvent form a two-phase system; at least one of the first and second reactive monomers containing concavity, where concavity-containing monomers are molecules comprising at least one spiro-contorted centre, at least one bridged ring moiety and/or sterically congested single covalent bonds around which there is restricted rotation; the reaction of the first and second reactive monomers results in the in-situ formation of a covalently cross-linked 3-dimensional polymeric network possessing intrinsic microporosity, said intrinsic microporosity being defined as a continuous network of interconnected intermolecular voids, said voids arising from the structure of the first and second monomers; and the reaction of the first and second reactive monomers results in a separating layer forming on the support membrane to give a composite membrane; (i) a second solvent for the second reactive monomer; (ii) a second reactive monomer; the first solvent and the second solvent form a two-phase system; at least one of the first and second reactive monomers containing concavity, where concavity-containing monomers are molecules comprising at least one spiro-contorted centre, at least one bridged ring moiety and/or sterically congested single covalent bonds around which there is restricted rotation; the reaction of the first and second reactive monomers results in the in-situ formation of a covalently cross-linked 3-dimensional polymeric network possessing intrinsic microporosity, said intrinsic microporosity being defined as a continuous network of interconnected intermolecular voids, said voids arising from the structure of the first and second monomers; and the reaction of the first and second reactive monomers results in a separating layer forming on the support membrane to give a composite membrane; (c) optionally, after a reaction period, capping the unreacted groups of the separating layer with functional groups to modify the surface chemistry; (d) after a reaction period, immersing the resulting composite membrane into a quench medium; and (e) optionally, curing the membrane with temperature or microwaves for a given time.