Polyethersulfone (PES)/sulfonated poly(arylene ether sulfone) (SPAES) blend membranes were prepared using a phase separation method. The miscibility of the PES/SPAES blend solution was confirmed through a differential scanning calorimetry (DSC) and the membranes containing a maximum of 3 wt.% of SPAES were selected for studying the effect of the sulfonic acid groups on the membrane performance. The increase of the solution viscosity through the addition of SPAES strongly affected the thickness of the top layer. The thermal treatment of the blend membranes caused the densification of the skin layer and reduced the pore sizes on the top layer. The changes on the membrane surface depending on the SPAES concentration and heating temperature were observed using an atomic force microscope (AFM). The roughness and contact angle decreased with an increase in the SPAES concentration in the membranes, while increases in the heating temperature increased the roughness and contact angle through membrane shrinkage. The increase of the SPAES concentration and heating temperature resulted in a decrease of the water permeability but a higher salt rejection. While the salt rejection gradually increased with the increase of the SPAES concentration, the thermal treatment only significantly affected the blend membrane with a high SPAES concentration. (C) 2012 Elsevier B.V. All rights reserved.