This is the simplest type of carbohydrate, commonly known as a simple sugar, which acts as a building block for larger carbohydrates such as starches.

Monosaccharides are the simplest carbohydrate compounds, having between three and seven carbons. They are monomers, that is, they cannot be hydrolyzed. They usually have sweet taste and are always soluble in water. The most popularly known are pentoses and hexoses.

Most monosaccharides in aqueous solution are cyclic (ring-shaped). A reaction occurs between the carbonyl group and an OH group of the molecule itself, forming homoacetal compounds when it has aldehyde function and homocetals when it has ketone function.