The reaction is catalysed in the reverse direction. This activity is usually associated with butanediol dehydrogenase activity (EC 1.1.1.4 or EC 1.1.1.76). While the butanediol dehydrogenase activity is reversible, diacetyl reductase activity is irreversible. This enzyme has been reported in the yeast Saccharomyces cerevisiae [1,2]. Different from EC 1.1.1.304, diacetyl reductase [(S)-acetoin forming].