Fermentative production of solvents (acetone, butanol, and ethanol) by Clostridium acetobutylicum is generally a biphasic process consisting of acidogenesis and solventogenesis. We report that the biphasic metabolism of C. acetobutylicum could be changed by oxidoreduction potential (ORP) regulation. When using air to control the ORP of the fermentation broth at -290 mV, an earlier initiation of solventogenesis was achieved. Solvent production reached 25.6 gl(-1) (2.8 g acetone l(-1), 16.8 g butanol l(-1), 6.0 g ethanol l(-1)), a 35% increase compared with the ORP uncontrolled process. Metabolic flux analysis revealed that there was a general increase of the central carbon flux in the first 24 h of fermentation when ORP was controlled at -290 mV, compared with the control. Specifically, the solvent ratio (acetone: butanol: ethanol) was changed from 25:64:11 to 11:66:23 at ORP level of -290 mV, which might have resulted from the rigidity at acetyl-CoA node and the flexibility at acetoacetyl-CoA and butyryl-CoA nodes in response to ORP regulation.