Reduction of β-keto-diphenylphosphine oxide (S)-4 derived from (S)-mandelic acid with NaBH 4 in methanol at −78°C gave the anti-3-diphenylphosphinoyl-1,2-diol 6, whereas the reduction of (S)-4 with Zn(BH 4 ) 2 in THF at −78°C afforded the syn-1,2-diol 5, both in high yield and excellent diastereoselectivity. This procedure represents a new approach to the diastereoselective synthesis of diphenylphosphinoyl diols and an example of highly diastereoselective 1,2-induction. A theoretical model to explain the direction and degree of asymmetric induction is proposed.