Poly(phthalazinone ether sulfone) (PPES) ultrafiltration(UF) membranes were prepared by the phase inversion method using N-methyl-2-pyrrolidone (NMP)/tetrahydrofuran (THF) as solvents. Four non-solvent additives (NSAs) having different affinity with the cosolvent THF, in terms of ethylene glycol methyl ether (EGME), diethelene glycol (DegOH), dimethyl sulfoxide (DMSO) and gamma-butyrolactone (GBL), were used to adjust morphologies and separation performances of the PPES membranes. It was shown that the compatibility difference of components in the mixed solvent system might have an important influence on the final performance of PPES fibers. The compatibility difference of the components in the mixed solvent system had been proved by the measurement of PPES intrinsic viscosity with different NSA approaching ratios in the polymer solution. And the compatibility difference of the components in the polymer solution had been evaluated by the analysis of the solubility parameter theory. The bigger compatibility difference of components in the mixed solvent system and the lower diffusion coefficient of NSA in the quenching water bath were benefit for the formation of membrane with a high molecule weight cut-off (MWCO). (C) 2010 Elsevier B.V. All rights reserved.