The purpose of this work was to study the yeast enzymes involved in the removal of diacetyl and structurally related compounds in beer. Yeast alcohol dehydrogenase (ADH) catalyzes the reduction of three vicinal dicarbonyl compounds found in beer: methyl glyoxal. diacetyl, and 2,3-pentanedione. The pH optimum for these activities is 6–6.5. While ADH uses these compounds with equal facility, they are poor substrates compared to acetaldehyde. ADH does not interconvert acetoin and 2,3-butanediol or acetol and 1,2-propanediol. Yeast has other enzyme activity which reduces acetoin, acetol, diacetyl, and 2,3-pentanedione. Thus, the latter two compounds, which are important flavor-influencing vicinal diketones, are reduced by more than one enzyme. This other enzyme activity is also capable of oxidizing 2,3-butanediol and 2,3-pentancdiol. The oxidation (dehydrogenation) of butanediol is catalyzed by more than one enzyme.