Polyhydroxy aldehydes are molecules with more than one hydroxyl group (−OH), and a carbonyl group (C=O) either at the terminal carbon atom (aldose) or at the second carbon atom (ketose). The carbonyl group combines in aqueous solution with one hydroxyl group to form a cyclic compound (hemi-acetal or hemi-ketal).  Monosaccharides are polyhydroxy aldehydes