The oxidation of volcanic SO2 to sulphate in the stratosphere is investigated for an anomalously large volcanic sulphur loading (∼200 Mt) similar to that generated by the Tambora eruption in 1815. Model calculations suggest that stratospheric OH levels can be severely reduced by the gas-phase oxidation of SO2. One implication of this is that dense volcanic SO2 clouds may last substantially longer than previously thought. SO2 oxidation is also found to lead to significant stratospheric dehydration because approximatively three molecules of H2O are consumed for each molecule of SO2 converted to sulphate. Therefore it is possible to view major volcanic eruptions as a potential sink for stratospheric H2O.