Fermentation performance of 15 thermotolerant Saccharomyces cerevisiae in three musts from dried grapes at 25, 30, and 40 degrees brix was studied. When the osmotic strength was increased, the volatile acidity and the SO2 production in the wines also increased: in the must with 40 degrees brix, yeasts produce from 1.63 to 3.65 g acetic acid l(-1) and from 40 to 73.6 mg SO2 l(-1) due to osmotic stress. From 9.75 to 13.40 ethanol v/v production is observed in must at 30 degrees brix, whereas at 40 degrees brix there is a clear detrimental effect.