The effect of sulfonated poly(ether ether ketone) (SPEEK) in membrane formation and separation properties has been investigated in polysulfone(PSU)/SPEEK/N-methyl-2-pyrrolidinone (NMP) systems. Charged ultrafiltration/nanofiltration membranes were obtained reliably in the range of 0.5-5 wt.% SPEEK in the polymer blend. All PSU/SPEEK blend membranes had substantially higher water flux, salt rejection, porosity and greatly reduced particle adhesion compared to the PSU base membrane, Further, all of these properties varied systematically with variation of SPEEK content. Reproducibility and stability of the membrane properties was excellent. Port: sizes determined from dextran retention data and AFM measurements showed reasonable agreement. Membranes with 5 wt.% SPEEK demonstrated excellent overall properties. Such membranes had very high permeability, 22.6 +/- 1.6 x 10(-11) m(3) s(-1) N-1, 0.999 fractional rejection of 4000 Da dextran, 0.65 rejection of 0.001 M NaCl, and only 0.75 mN m(-1) adhesion of a 4 mum silica particle. Such membranes are very promising for scale-up of production and testing on real process streams.