Soluble copolysulfoneimides were synthesized by thermal two-step method in solution of N-methyl-2-pyrrolidone. The used aromatic diamines were bis[4-(3-aminophenoxy)phenyl]sulfone (BAPS-m) and 3,3'-diaminosulfone, and dianhydrides were pyromellitic dianhydride, 4,4'-oxyphthalic anhydride, and 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride. The molar ratio of diamines was changed to reduce the content of BAPS-m. The thermal and mechanical properties of polyimides were investigated. The polyimide ultrafiltration membrane with molecular weight cut-off of 10 kDa could be successfully prepared by phase-inversion method. Various solvent (water, alcohols, acetone, and hexane) fluxes were measured to investigate solvent-resistance and membrane behavior during solvent permeation. The activation energy relationship between hexane flux and viscosity with temperature was also studied.