A method of storing gas comprises providing a recipient for receiving the gas and providing a porous gas storage material. The gas storage material comprises a cross-linked polymeric framework and a plurality of pores for gas sorption. The cross-linked polymeric framework comprises aromatic ring-containing monomeric units comprising at least two aromatic rings. The aromatic ring-containing monomeric units are linked by covalent cross-linking between aromatic rings to form a stable, rigid nanoporous material for storing the gas at pressures significantly greater than the atmospheric pressure, for example in excess of 100 bar. A possible application is the storage and transportation of compressed natural gas.