A new series of positively charged asymmetrical membranes were prepared by in situ amination and phase inversion in which the dimethylformamide (DMF) solution of trimethylamine (TMA) and brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) was cast and immerged into an ethanol coagulation bath. The membranes were characterized in terms of pure water flux, rejection, streaming potential and SEM. The effect of the amount of amine and evaporation time on the membrane performance and structure were investigated, respectively. The separation properties of the membrane were exemplified with gelatin solution of different pH values. The results showed that with an increase in amount of amine, the flux of pure water enhanced. But the rejection to gelatin depended on both the electrostatic interaction and membrane microstructure. For the membrane at the same ratio of BPPO to TMA, the rejection decreased with an increase in pH value due to electrostatic effect; while for the membrane of different ratio of BPPO to TMA, the rejection increased to some extent at all the considered pH values with an increase in ratio of BPPO to TMA due to sieve effect. In addition, shorter evaporation time was beneficial to the flux and negative to the rejection. (C) 2004 Elsevier B.V. All rights reserved.