The net production and consumption of acidity by Desulfovibrio fructosovorans, growing with lactate as the carbon and energy source, were measured in an unbuffered medium in a pH-controlled bioreactor. At alkaline pH (7.2 and 8.5), net acidity production was measured. At pH 6.0, acidity consumption was obtained, although bacterial growth was not observed. These observations are in good agreement with theoretical predictions emphasizing the key role of H+ ions in the relationship between the metabolism of sulphate-reducing bacteria and microbially influenced corrosion.