The growth inhibition of Moorella sp. HUC22-1 by undissociated acetic acid was analyzed using a non-competitive inhibition model coupled with a pH inhibition model. In the cells grown on H2 and CO2, the inhibition constant, K(p) of the undissociated acetic acid was 6.2 mM (164 mM as the total acetate at pH 6.2, pKa = 4.795, 55 degrees C), which was 1.5-fold higher than that obtained in cells grown on fructose. When a pH-controlled batch culture was performed using a fermentor at pH 6.2 with H2 and CO2, a maximum of 0.92 g/l of dry cell weight and 339 mM of acetate were produced after 220 h, which were 4.4- and 6.8-fold higher than those produced in the pH-uncontrolled batch culture, respectively. In order to reduce acetate inhibition in the culture medium, a repeated batch culture with cell recycling was performed at a constant pH with H2 and CO2. At a pH of 6.2, the total acetate production reached 840 mmol/l-reactor with 4.7 mmol/l-reactor of total ethanol production after 420 h. When the culture pH was maintained at 5.8, which was the optimum for ethanol production, the total ethanol production reached 15.4 mmol/l-reactor after 430 h, although the total acetate production was decreased to 675 mmol/l-reactor.