The characteristics of the phases inversion process were studied during the formation of an asymmetric polyetherimide (PEI) membrane, whlie considering the effect of zinc chloride (ZnCl₂) as an additive. Generally, the formation process of a polymeric membrane by phase inversion can be explained in terms of two sets of factors; the thermodynamic properties of the casting system and the kinetics involved in the exchange of the solvent and the nonsolvent. When the inorganic salt, ZnCl₂, was introduced to the casting solution as an additive, it had a slight effect on the thermodynamic and kinetic data of the membrane-forming system, leading to a change in the membrane morphology and permeation performance. The addition of ZnCl₂ increased the viscosity of the casting solution and decreased the coagulation value while slightly decreasing the precipitation rate. It was also observed that the increased ZnCl₂ content in the casting solution led to the drop of the hydraulic permeability of the membrane, while increasing the solute rejection rate. The SEM observations showed that the top-layer of the membrane became thicker and denser and the size of the macrovoid in the sublayer became larger with increasing ZnCl₂ contents in the casting solution.