Carbon monoxide is a poisonous gas that results from the automobile emission, gasification of biomass and sewage sludge. The conversion of carbon monoxide as the gaseous substrate to acetic acid by Clostridium aceticum is an alternative usage route to curb the CO problem at relatively low cost. Mixed levels of full factorial design have been conducted to study the effects of CO partial pressure ranging from 1.40 to 2.02 atm and the fermentation time (0-120 h) over the cell density, CO residue and acetic acid concentration in the batch system. The applied mixed gas composition were 4% H-2: 18% Argon: 78% CO. The response surface plot computed from experimental design was employed to optimize the process operating conditions for maximum cell density, minimum CO residue and maximum acetic acid productivity. The results from the surface plot shows that acetic acid fermentation is best operated at 1.40 atm CO partial pressure and at 48 h fermentation times in order to achieve process optimization. (c) 2006 Elsevier Inc. All rights reserved.