In this study, Cloud point experiments and Flory Huggins’s theory for polymer solutions are used to construct ternary phase diagrams for systems of Water/Solvent(s)/Polysulfone. The different solvents used are NMP, THF, and their mixtures, with Ethanol being optionally used as a nonsolvent additive. The ability of the F-H theory to predict the binodal phase boundaries with reasonable accuracy for both pure solvent and solvent mixtures is established. The binodal curve was distinct for each solvent and influences the precipitation path followed in phase inversion. The effect of this on the membrane structure was studied using scanning electron microscopy, and in turn, the influence of structure on membrane performance was studied in a permeation cell at a feed pressure of 3 bar. Membranes cast with a mixture of NMP-THF with EtOH as a nonsolvent showed the best performance with CO2 permeance of 7.07 GPU and CO2/CH4 selectivity of 20.77.