Ultrafiltration membranes used for drinking water treatment need to have high rejection for microbial species. The effect of various inorganic halides added to a casting solution of poly(acrylonitrile) in N,N-dimethyl formamide was examined by measuring changes in the intrinsic viscosity, concentrated solution viscosity and the ternary phase boundary. Both mono-and divalent salts decreased polymer-solvent affinity; the higher viscosity in the case of polymer solutions containing multivalent salts was attributed to increased interaction between polymer chains. The resulting membrane structure was characterized by scanning electron microscopy and bubble point measurements. The addition of di- and trivalent salts resulted in membranes with a mode pore size similar to the membrane prepared without any additive; but significantly fewer number of large pores/defects and consequently higher rejection for E. coli.