In the present work, the effect of additive, polyethylene glycol 600 (PEG 600), on the ultrafiltration performance of the polysulfone (PSf)/sulfonated poly(ether ether ketone) (SPEEK) blend flat sheet membranes prepared in a polar solvent of N,N'-dimethylformamide (DMF) by phase separation process was investigated. The effects of additive concentration on the ultrafiltration performance such as compaction, pure water flux, water content, hydraulic permeability and morphology were investigated. The addition of PEG 600 in the dope solution increases the exchange rate of additive and non-solvent during the membrane formation process, resulting in the appearance of the macrovoids formation as visualized under microscope. The pure water flux and Equilibrium water content of the blend membranes were increased while hydraulic permeability was decreased. (c) 2009 Elsevier B.V. All rights reserved.