The pore-forming behavior in polyimide membranes was investigated by varying the amounts of N,N＇-dimethylacetamide (DMAc, wt.% versus water as a nonsolvent) in coagulation baths. The pore-forming behavior was explained by phase diagrams, coagulation values, diffusion coefficients of nonsolvents, and solubility parameters. The nonsolvent induced phase inversion (NIPI) process prepared the asymmetric polyimide membranes. Increasing the amount of DMAc delayed the demixing phase inversion, increased the stability of the polymer ternary system, and reduced the pore specific volume of sub-layers and the pore size in the skin layers. The diffusion coefficient, D-e, of the nonsolvents into the PAA solutions were in the range of 0.39-2.97 x 10(-5) cm(2)/s. Adding DMAc in a coagulation bath resulted in a small difference of the solubility parameter, Delta delta(S),(NS), and also making the delayed demixing easier. A less porous structure is formed by strong interactions between solvent and nonsolvent; nonsolvent penetration is delayed in the polymer solution. The sub-pore shape in polyimide membranes, which were a finger-like structure, did not change as the amount of DMAc increased. Overall, the DMAc in the pore-forming process had significant effects on the membrane morphology. The choice of solvent as the role of counter additive to a nonsolvent could be an important factor in the membrane preparation.