Chemical synthesis of a deuterated primary carbinol, L-threose-4-d (4S), has been effected with virtually complete stereoselectivity. It involves deuteride reduction of 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide, conversion of the resulting 1,5-anhydride-1-d into the corresponding 4-deoxy-6-aldehydo-hex-4-ene derivative, and then ozonolysis, without deuterium scrambling. In this way, C-4 and stereoselectively- deuterated C-1 of the hexose have been interconverted to become, respectively, C-1 and stereoselectively- deuterated C-4 of the L-threose molecule. This study also furnishes support for previous evidence that D-galactose oxidase catalyzes removal of the pro-S 6-hydrogen atom of D-galactose.