Osazone reaction is possible only when a free carbonyl (aldehyde or ketone) group is present.

A monosaccharide or reducing disaccharide is first mixed and heated with phenylhydrazine in acetic acid to form sugar-phenylhydrazine, which then reacts with more reagents to form particular osazone-forming characteristic crystals of specific shapes. This test is very important to distinguish and identify the different reducing sugars. Glucose, fructose, and mannose form the same osazones, that is, shape (they form the same needle-shaped crystals). Maltose forms sulphur-shaped crystals and lactose form a ball-shaped crystals.