A simplified method to prepare polysulfone (PSf)/polyaniline (PANI) nanocomposite membrane was proposed. With this method, aniline was polymerized using ammonium peroxydisulfate as oxidant and N-methyl-2-pyrrolidone (NMP) as solvent, which was also the solvent of PSf. The mixture after aniline polymerization containing PANI nanospheres, PANI oligomers, NMP and inorganic ions, named PANI/NMP, was added directly into the solution consisting of PSf and NMP without any post-processing to form membrane casting solution. Then, PSf/PANI nanocomposite membranes were prepared via immersion phase inversion process using the casing solution. This method is called in situ blending method. PANI oligomers were soluble in both water and organic solvent while PANI nanospheres were dispersed in both water and organic solvent. The behaviors of PANI oligomers and PANI nanospheres during membrane formation were investigated and analyzed. A large portion of PANI oligomers and a small portion of PANI nanospheres would be leached out of the casting film during the phase separation and act as pore formers, inducing the enhancement of membrane porosity and surface pore size. A large portion of PANI nanospheres and a small portion of PANI oligomers residing in the prepared membranes favored the enhancement of membrane hydrophilicity, breaking strength and thermal stability. Compared with PSf membrane, PSf/PANI nanocomposite membranes obtained with this method had more hydrophilic surface, higher porosity, larger surface pore size, less acicular pores, thicker skinlayer, wider pores beneath the skinlayer, better vertically interconnected finger-like pores and less macrovoids. Pure water fluxes of PSf/PANI nanocomposite membranes were 1.7-4.1 times that of PSf membrane and BSA rejections of the membranes were all above 96%. During BSA ultrafiltration, PSf/PANI nanocomposite membranes showed slower flux decline rate and higher flux recovery ratio values after simple water flushing than PSf membrane. (C) 2011 Elsevier B.V. All rights reserved.