In the present work, PMMA membranes were prepared by wet phase immersion methods to improve their gas fluxes. It is found that different membrane structure can be obtained by using different nonsolvent-solvent pairs. To completely describe the membrane formation process, the nonsolvent-solvent miscibility and the interfacial polymer concentration in casting solution should be considered accompanied by the ternary phase diagram. A simplified solution-diffusion model was developed to estimate the interfacial polymer concentration. In addition, the effects of adding solvent into the coagulation bath and adding nonsolvent into the casting solution are discussed.