Permeation characteristics were investigated using forward osmosis for the dewatering of VFAs solution. A solution of VFAs produced by anaerobic mixed-culture fermentation can be used as a raw material to produce mixed alcohol and microbial biodiesel. A 35 g/L surrogate solution of 6:1:3 ratio (acetic, propionic, and butyric acids) was prepared. Under membrane orientation, the water flux, reverse salt flux, and rejection of VFAs in the FO mode were higher than those in the PRO mode. Both the flux and rejection rate were pH-dependent for the VFA solution. The flux increased at a lower pH, while the rejection increased at a higher pH. At pH 8, an optimum performance of 97% rejection and a 11.10 L/m(2) h water flux was obtained. The water flux was also influenced by the temperature, concentration of the draw solution, and type of draw solute, but the rejection of VFAs remained nearly constant. In a fermentation broth, the flux behaviors were different from those in a chemical solution because the remaining nutrients and by-products increased the osmotic pressure in the feed solution. The flux behavior findings obtained under various experimental conditions provide clues to the mechanisms and parameters for the dewatering of VFAs solution.