Polysulfone (PSf) and five fluorinated polyimides were tested for their solubility with 24 organics or their mixtures in order to select a solvent suitable for developing a composite membrane consisting of an inexpensive substrate with an expensive selective layer of high-performance fluorinated polyimides for CO2/CH4 separation. The experimental results show that only nitromethane can dissolve 6F-Durene and 6F Durene-mPDA but not dissolve PSE The manufacture of a 6F-Durene composite membrane using PSf as a substrate was attempted with the dip-coating method, which has not been previously reported. The PSf/6f-Durene composite membrane after repair with silicon rubber was defect free. There is a potential to manufacture a defect-free fluorinated polyimide composite hollow-fiber membrane.