Propylene glycol (IUPAC name: propane-1,2-diol) is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. As it contains two alcohol groups, it is classed as a diol. It is miscible with a broad range of solvents, including water, acetone, and chloroform. In general, glycols are non-irritating and have very low volatility.

A diol is a chemical compound containing two hydroxyl groups. An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting groups of carbonyl groups, making them essential in synthesis of organic chemistry.