Polyethersulfone (PES) nanofiltration membranes were prepared using a simple dry-jet wet spinning technique with different contents of sulfonated poly(ether ether ketone) (SPEEK) ranging from 0 to 4 wt %. The structural parameters (r(p) and A(k)/Delta x) and electrostatic properties (xi and X) of the blend membranes were deduced by employing the combination of irreversible thermodynamic mode steric hindrance pore (SHP) model and Teorell-Meyyer-Sievers (TMS) model. The modeling results obtained have been analyzed and discussed. The mean pore radius and pore size distribution of the blends were also determined based on the theoretical models. The results showed that pore radius increased with increasing the concentration of SPEEK from 0 to 2 wt % but decreased with a further increase in SPEEK content. The water flux however showed a systematically increase with increasing SPEEK content. The SPEEK also showed significant effect on membrance electrical properties. Both effective charge density and ratio of effective charge density to electrolyte solution increased with increasing concentration of SPEEK in the dope solution reaching a value of -21.02 and -2.29 respectively. The pore radius which was determined by using different transport models has also been analyzed and discussed. It is found that the addition of SPEEK into dope solution is one of the paramount parameters in developing the negatively charged nanofiltration membrane with enhanced water flux while retaining the pore radius in the nanometer rnage. (C) 2009 American Institute of Chemical Engineers AIChE J. 55: 2081-2093 2009