BACKGROUND: Syngas and some waste gases are composed mainly of carbon monoxide, carbon dioxide and hydrogen, which can be used by some acetogenic bacteria to produce ethanol, butanol or hexanol, and represents an attractive alternative to the conventional ABE (acetone-butanol-ethanol) fermentation. Experiments were carried out in bioreactors under different conditions (pH 5.75 and 4.75) with continuous supply of a mixture of CO/CO2/H-2/N-2 converted by Clostridium carboxidivorans into hexanol, butanol and ethanol (H-B-E fermentation). RESULTS: Applying different pH control strategies will affect the syngas fermentation pattern, among others in terms of bioconversion rates as well as final concentrations of acids and alcohols. The highest concentrations of alcohols were obtained at pH 5.75, i.e. 2.7 g L-1 ethanol, 1.9 g L-1 butanol and 0.85 g L-1 hexanol, whereas the maximum production rates were observed at pH 4.75, reaching 0.048 g-ethanol h(-1) g(-1)-biomass, 0.037 g-butanol h(-1) g(-1)-biomass, and 0.026 g-hexanol h(-1) g(-1)-biomass. However, a low pH negatively affects growth and acids production in the first metabolic step, with lower growth and acids production at pH 4.75 than 5.75. Growth rates reached 0.0057 h(-1) and 0.072 h(-1), respectively, at pH 4.75 and 5.75. CONCLUSIONS: Maintaining initially a higher pH of 5.75 allows accumulating higher concentrations of acids than when natural acidification takes place. Such higher concentrations of acids allow the production of higher amounts of alcohols as end metabolites, showing the importance of pH on bioconversion and biomass growth. (C) 2017 Society of Chemical Industry