Fermentation of CO or syngas offers an attractive route to produce bioethanol. However, during the bio-conversion, one of the challenges to overcome is to reduce the production of acetic acid in order to minimize recovery costs. Different experiments were done with Clostridium autoethanogenum. With the addition of 0.75 mu M tungsten, ethanol production from carbon monoxide increased by about 128% compared to the control, without such addition, in batch mode. In bioreactors with continuous carbon monoxide supply, the maximum biomass concentration reached at pH 6.0 was 109% higher than the maximum achieved at pH 4.75 but, interestingly, at pH 4.75, no acetic acid was produced and the ethanol titer reached a maximum of 867 mg/L with minor amounts of 2,3-butanediol (46 mg/L). At the higher pH studied (pH 6.0) in the continuous gas-fed bioreactor, almost equal amounts of ethanol and acetic acid were formed, reaching 907.72 mg/L and 910.69 mg/L respectively. (C) 2015 Elsevier Ltd. All rights reserved.