A series of Fe3O4-PES ultrafiltration membranes with different mass ratios of Fe3O4 and PAA were prepared from suspensions, using the phase inversion process. The suspensions consisted of polyether sulfone (PES), dimethyl formamide, polyacrylic acid (PAA), and ferrosoferric oxide (Fe3O4). The separation properties of ultrafiltration membranes with different Fe3O4/PAA mass ratio were investigated by a cross-flow experimental system. The Fe3O4/PAA mass ratio had little effect on the rejection of membranes to BSA. However, the pure water flux had a slight decline and then rised rapidly with the increase of Fe3O4/PAA mass ratio. An interesting phenomenon observed was that the Fe3O4 particles could diffuse into the nonsolvent bath during the formation of membrane, and the amount of Fe3O4 extracted into the nonsolvent bath nearly kept a constant mass ratio to PAA, even if the Fe3O4/PAA proportion was changed. The reasons of this interesting phenomenon were investigated. This result indicates that modified inorganic fillers may be used as the pore-forming agent to prepare the porous membranes like the template leaching method. At the same time, this method does not use any strong acid or base. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013