The present study was undertaken to investigate the effects of low molecular weight inorganic additives(electrolytes) contained in precipitant on membrane performance. The system studied was polysulfone(13 wt%)/ dimethylformamide / water(or water+additive) and additives employed were NaCl, NaOH, KCI, LiCl. CaCl₂, etc. Asymmetric ultrafiltration membranes were manufactured from the immersion precipitation method by varying types and amounts of additives and tests on the structures and performances of the resultant membranes were made by measuring water content, porosity, membrane thickness, pure water flux, dextran rejection (available only for NaCl), and SEM photographs. In addition, experiments on solvent outflow time and cloud point (phase separation) were carried out for elucidation of membrane formation mechanism. For the effective discussion of the "additive"effect, we divided the additive concentration range into three regions ( low concentration, 03) and analyzed the experimental data in each ragion by taking account of the specific nature(e.g., the "salting-out" effect) of salt, decrease in water activity, increase in density difference between cast solvent and precipitant, variation in viscosity and mutual miscibility of liquids, etc, resulting from the presence of additives, which affect equilibrium thermodynamics and kinetic aspect during membrane formation process.