Flat sheet poly(phthalazinone ether sulfone ketone) (PPESK) ultrafiltration membranes were prepared. The effects of PPESK concentration, solvents, additives and exposing time on the structure and performance of PPESK ultrafiltration membranes were investigated in more detail. The optimal preparation conditions were: 12 wt.% PPESK, NMP/DMAc mixed solvent (mass ratio = 1), 8 wt.% polyethylene glycol 400 (PEG400), 2.5 wt.% LiCl and 5 s exposing time. Under these conditions, the pure water flux and the rejection of gamma-globulins were 1139L/m(2) h and 93.7% at the operation pressure of 0.1 MPa, respectively. Scanning electron microscope (SEM) micrographs showed that spongy structure could be formed while finger-like structure could be suppressed due to the longer exposing time or higher LiCl concentration. (c) 2005 Published by Elsevier B.V.