The effects of annealing on the microstructure and performance of cellulose acetate (CA) forward osmosis (FO) hollow fiber membranes have been studied. The mean pore radius decreases from 0.63 nm to 0.39, 0.36, 0.30 and 0.30 upon annealing at 70, 80, 85 and 90 degrees C, respectively. The density of free volume in the depth range of 0.31-1.33 mu m significantly decreases with depth with annealing at 80 degrees C. Annealing causes the shrinkage of both the voids between nodule aggregates and the spaces between intra-molecular chain segments. At depths greater than 0.31 mu m, the amount of large cavities increases for all the membranes and its increase are greater for the membranes annealed at 85 and 90 degrees C. The free volume radius and total free volume at a depth of 0.1 mu m increase with increasing annealing temperature. The amount of free volume at 0.1 mu m depth slightly decreases upon annealing at 70 and 80 degrees C but increases upon annealing at higher temperatures. In the pressure-retarded osmosis (PRO) process, the CA membrane annealed at 80 C exhibits the highest water flux of 30.98 L m(-2) h(-1) (LMH) with a salt leakage of 0.37 g m(-2) h(-1) (gMH) using 2.0 M MgCl(2) as the draw solution. (C) 2010 Elsevier B.V. All rights reserved.