Aims: The purpose of this work was to study the involvement of micro-organisms, which develop together with Botrytis cinerea on grapes, in the SO2 binding power of musts. Methods and Results: Yeasts and bacteria were involved. Most bacteria were acetic acid bacteria, mainly of the Gluconobacter genus. Unlike oxidative yeasts, Gluconobacter produce gluconic acid (in balance with delta -gluconolactone) from glucose, 5-oxofructose from fructose and dihydroxyacetone from glycerol. Production of carbonyl compounds from other sugars and polyols was not detected or was very weak. Conclusions: Acetic acid bacteria are responsible for the increases in SO2 binding power of musts from botrytized grapes by oxidizing the three main sugars of these grapes. Significance and Impact of the Study: Up to 80% of the SO2 binds with products of Gluconobacter which easily grow an 'botrytized' grapes. Depending on climatic conditions, some vintages are particularly difficult to stabilize.