Commercially available invertase (β-d-fructofuranoside fructohydrolase, E.C. 3.2.1.26) preparations fromSaccharomyces cerevisiae are reversibly deactivated by high ethanol concentrations and by NaCl concentrations that occur in industrial molasses media. A significant difference in stability was found between the invertase activity of Baker's yeast and of a highly specialized temperature-, ethanol-, and osmo-tolerant strain of S. cerevisiae. Deactivation of invertase is accompanied by dissociation of the glycosylated enzyme into subunits and is responsible for the limited maximum ethanol concentration of 80 g l - 1 , which is obtained in industrial yeast fermentations based on molasses.