Diastereoselective reduction of enantiomerically pure polymer is described. Reduction of poly((S)-1-oxo-2-methylpropylene) [(S)-1] with tetrabutylammonium borohydride (Bu4N . BH4) provided the corresponding polyol, poly(l-hydroxy 2-methylpropylene) (2), in a quantitative yield. The absolute configuration of the local structure was determined by the unambiguous synthesis of four diastereomers of 3-methyl-2,5-hexanediol. The newly formed asymmetric carbon was controlled to S-configuration with SIR ratio of 70/30. The S-selectivity contradicts to the conventional Cram-selectivity with which R-configuration is predicted from (S)-1. The origin of the selectivity is discussed by comparing the results with the reduction of a model molecule, 3-methyl-2,5-hexanedione.