The relationship between the characteristics of the polymer dope solution and the skin formation mechanism as well as the performance of the asymmetric membrane has been investigated. The solution characteristics have been studied on the polysulfone (PSf) dope solution as a function of the concentrations of both polymer and the cosolvent, ethyl acetate (EA), by dynamic light scattering. An anomalous light scattering was observed at small angles in both PSf/NMP and PSf/NMP:EA (6:4 by weight) solutions, indicating the inhomogeneity of the dope solutions. In the case of the PSf/NMP:EA (6:4) solution, an integrally skinned asymmetric membrane without defects having high gas selectivity was obtained while the membrane from the PSf/NMP solution had a defective skin. The scattered light intensity of the dope solution of PSf/NMP:EA (6:4) increased with the aging time while no notable change was observed in the PSf/NMP solution. The characteristics of the solution affect the final morphology of the membrane, particularly when phase separation occurred significantly before the immersion into the gelation medium.