Paraldehyde is the cyclic trimer of acetaldehyde molecules. A colourless liquid, it is sparingly soluble in water and highly soluble in alcohol.
Paraldehyde slowly oxidizes in air, turning brown and producing an odour of acetic acid. It quickly reacts with most plastics and rubber. Paraldehyde was first synthesized in 1829 by Wildenbusch. It has uses in industry and medicine.