Sulfonated poly(ether ether ketone) (SPEEK), a strong polyelectrolyte, was investigated as an additive in polysulfone (PSU)/SPEEK/N-methyl-2-pyrrolidinone (NMP) systems. Membrane characterisation was carried out using filtration studies and by atomic force microscopy (AFM). The effect of SPEEK in the charged membrane formation was investigated through thermodynamic and viscosity studies and correlated with the structure and properties of the resulting membranes. A "Polyelectrolyte behaviour" of SPEEK was observed for the first time in the membrane forming systems. Charged ultrafiltration/nanofiltration membranes prepared from 20 wt.% PSU solutions with the ratio of SPEEK/PSU in the range of 0.005-0.05, have substantially increased permeability, salt rejection, porosity, and a greatly reduced fouling tendency compared to the base PSU membrane. The charged membranes offer the possibility of effective removal of humic substances from surface waters in a highly efficient manner.