Biochemical methods, usually microbial or enzymatic, are suitable for the production of unnatural or rare monosaccharides. D-Arabitol was produced from D-glucose by fermentation with Candida famata R28. D-xylulose can also be produced from D-arabitol using Acetobacter aceti IFO 3281 and D-lyxose was produced enzymatically from D-xylulose using L-ribose isomerase (L-RI). Ribitol was oxidized to L-ribulose by microbial bioconversion with Acetobacter aceti IFO 3281; L-ribulose was epimerized to L-xylulose by the enzyme D-tagatose 3-epimerase and L-lyxose was produced by isomerization of L-RI. L-ribose and L-arabinose were prepared biochemically from ribitol by oxidation using Acetobacter aceti IFO 3281 and isomerization using L-RI and L-arabinose isomerase (L-AI), respectively. Other pentoses can be produced as well by cell or enzyme bioconversions.