Ion fluxes of acetic acid and phosphate in the electrodialysis of a model fermentation broth were examined using a solution of acetic acid and potassium dihydrogenphosphate. As the molar fraction of the phosphate was increased the ion flux of the acetic acid decreased, which was as predicted theoretically. As the influent pH was increased the flux also increased notably, reached a maximum, and then decreased slightly. The maximum flux was obtained in a pH range from 5.2 (near pK(a)) to 6.6. The pH at the exit of the electrodialyzer became markedly alkaline when the influent pH was in the range of 6 to 7 in the absence of the phosphate. The pH range shifted to the alkaline side and the degree of alkalinity reduced as the molar fraction of phosphate increased. This may be ascribed to the buffering effect of the phosphate. It is concluded that the optimum pH for the electrodialysis of an organic acid fermentation broth is around (pKa + 1), where a relatively high ion flux with high current efficiency and a small pH increase at the exit of the electrodialyzer are established.