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This work explored the preparation (both reproduced and novel), characterisation and modification of polymers of intrinsic microporosity (PIMs) using the first member of the series, PIM-1, as a representative example. The preparation was carried out using tetrafluoroterephthalonitrile (TFTPN) and tetrachloroterephthalonitrile (TCTPN) along with 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI), and compared the products from each synthetic route to assess which would be most appropriate for scale up. Routes incorporating TCTPN were studied as a means of developing a more economically viable route to PIM-1. Purification techniques were also reviewed and it was discovered that repeat washes of PIM-1 using 1,4-dioxane removed the majority of low molar mass polymer in the shortest period of time. Techniques including triple detector GPC, NMR (solution and solid state), ATR-IR and reversible gas sorption were used to characterise the polymer structure and understand the free-volume element within the material. The effect of molar mass on membrane fabrication was also studied, with results showing that 60, 000 g mol-1 is the minimum molar mass (MW) required to form a free-standing flexible membrane. The way PIM-1 interacts with foreign systems was examined from two different perspectives. First, solubility studies were able to qualify how PIM-1 behaves in a range of common solvents and how this solubility changes with molar mass. Second, attempts to quantify the swelling of the polymer in lower alcohols and water were made. N2 sorption proved the most quantitative method and showed that the size of the alcohol has an effect on how much excess free volume is available after the polymer has been exposed to alcohol. ATR-IR showed that alcohols have different effects on different functional groups with red shifts observed for the ether linkage, a result most likely due to hydrogen bonding, and blue shifts observed for the nitrile group. Wide angle X-ray scattering (WAXS) showed three amorphous halos present in the PIM-1 structure, the positions of which did not change after exposure to alcohols and water. There was a change in intensity of these halos after the polymer had been exposed to various solvents. The PIM-1 polymer backbone was also successfully modified by replacing the nitrile group to give Thioamide PIM-1, Thiazole PIM-1, Amide PIM-1 and Amine PIM-1, in both powder and membrane forms. All of the modified polymers were successfully characterised using IR, NMR (solution state where soluble and solid state), WAXS and thermogravimetric analysis (TGA). CO2 and N2 sorption was carried out on all modified polymers and Amine PIM-1 showed the highest CO2 uptake (2.3 mmol g-1 at 0.7 bar, 6.3 mmol g-1 at 10 bar). This increase is most likely due to favourable interactions between the amine functionalities and the CO2 probe molecule. Organic molecules of intrinsic microprosity (OMIMs) are a novel class of discrete materials prepared using the similar chemistry used to prepare PIM-1, and were compared to the polymer from the perspective of gas sorption. All OMIMs examined showed similar N2 uptake to that measured for an aged sample of PIM-1, ranging from 595 – 624 m2 g-1 (excluding OMIM-2), and sample history was found to have a large effect on the sorption characteristics of the molecules from the perspective of both N2 and a CO2 probe molecules.