Stripping gas is generally used to improve hydrogen yields in fermentations. Since CO(2) is relatively easy to separate from hydrogen it could be an interesting stripping gas. However, a higher partial CO(2) pressure is accompanied with an increased CO(2) uptake in the liquid, where it hydrolyses and induces an increased requirement of NaOH to maintain the pH. This enhances the osmotic pressure in the culture by 30%, which inhibited the growth of Caldicellulosiruptor saccharolyticus. indications for this conclusion are: i) inhibition could almost completely be circumvented by reducing the bicarbonate through decreasing the pH (from 6.5 to 5.5), ii) Growth rates were reduced by 60 +/- 10% at an osmotic pressure of 0.218 +/- 0.005 osm/kg H(2)O independently of the stripping gas, iii) Increased extracellular DNA and protein concentrations were observed as a function of the osmotic pressure. In addition to growth inhibition, the increased sodium bicarbonate in the effluent will contribute to a negative environmental impact when applied at industrial scale. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All lights reserved.