The yeast mediated reduction of a range of -and -deutero substituted nitrostyrenes has been conducted in light petroleum in the presence of a small amount of water or D 2 O. NMR analysis of the products from these reactions has allowed the determination of the mechanism of this yeast reduction reaction. The results indicate that initially, a reversible non-stereoselective protonation occurs at the -centre, followed by stereoselective addition of hydride at the -position.