any of a class of organic compounds belonging to the alcohol family; in the molecule of a glycol, two hydroxyl (OH) groups are attached to different carbon atoms. The term is often applied to the simplest member of the class, ethylene glycol.

Ethylene glycol, also called 1,2-ethanediol, molecular formula HOCH₂CH₂OH, is a colourless, oily liquid possessing a sweet taste and mild odour. It is produced commercially from ethylene oxide, which is obtained from ethylene. Ethylene glycol is widely used as antifreeze in automobile cooling systems and in the manufacture of man-made fibres, low-freezing explosives, and brake fluid. Ethylene glycol and some of its derivatives are mildly toxic.

Propylene glycol, also called 1,2-propanediol, resembles ethylene glycol in its physical properties. Unlike ethylene glycol, however, propylene glycol is not toxic and is used extensively in foods, cosmetics, and oral hygiene products as a solvent, preservative, and moisture-retaining agent. Propylene glycol is manufactured in large amounts from propylene oxide, which is obtained from propylene.

Other important glycols include 1,3-butanediol, used as a starting material for the manufacture of brake fluids and of plasticizers for resins; 1,4-butanediol, used in polyurethanes and in polyester resins for coatings and plasticizers, and for making butyrolactone, a valuable solvent and chemical intermediate; 2-ethyl-1,3-hexanediol, an effective insect repellent; and 2-methyl-2-propyl-1,3-propanediol, made into meprobamate, a widely used tranquillizing drug.