Asymmetric gas separation membranes with dense and defect-free skin layer can be prepared by controlling the location of liquid-liquid demixing gap in a ternary phase diagram, and by controlling the residence time in the gel region. For this purpose, polymer solutions of polysulfone in NMP were immersed in polyethylene glycol (PEG) prior to transferring to a water bath. It was believed that a coating layer of PEG retards the liquid-liquid phase separation and controls the demixing of the solution. The investigations indicated that an optimum residence time in the PEG bath existed, which led to the formation of a defect-free, and consequently, a selective skin layer. The skin formation mechanism was discussed in terms of gelation and liquid-liquid phase separation. Also, nodular structure formation was interpreted in the view of spinodal decomposition and the second liquid-liquid phase separation in the water bath. In addition, Micro-Raman spectroscopy was used to investigate the composition change across the interface of the precipitated phase during phase inversion process. The difference in gelation process and liquid-liquid phase separation was observed spectroscopically. The results are incorporated into a discussion of the skin formation of asymmetric membrane.