Soluble polyimides were synthesized and characterized from two diamines and four dianhydrides by the two- and the one-step method. Most of the polyimides could be soluble by one-step method except alpha,alpha＇-bis(4-aminophenyl)-1,4-diisopropy benzene/3,3＇,4,4＇-benzophenonetetracarboxylic dianhydride system in limited organic solvents. Glass transition temperatures ranged from 186 to 233 degrees C and crystalline melt temperatures were not observed. All the soluble polyimides showed good thermal, mechanical, and electrical properties. The polyimides did not have crystalline structure and limited solubilities. The effective solvent had a medium dispersion component associated with weak polar and hydrogen components. The polymer from one-step polymerization had a narrower molecular weight distribution than the two-step method. Polyimide synthesized with 4,4＇-oxydiphthalic anhydride and bis[4-(3-aminophenoxy)phenyl] sulfone by two-step method could only be prepared by the typical phase inversion method. Other membranes except this polyimide membrane could not be prepared by the typical phase-inversion method because of poor solubility about polar solvents. The flux of this ultrafiltration membrane was very high, and this membrane could especially retain polymer having a molecular weight 20,000 to above 90%.