Polyethylene glycols (PEGs) are condensation polymers of ethylene oxide and water with the general formula H(OCH₂CH₂)_nOH. They are the most commercially important type of polyether. The low molecular weight compounds up to 700 are colorless, odorless viscous liquids with a freezing point from -10˚C (diethylene gycol), while polymerized compounds with higher molecular weight than 1,000 are wax like solids with melting point up to 67˚C. While PEGs with different molecular weights find use in different applications and have different physical properties (e.g. viscosity) due to chain length effects, their chemical properties are nearly identical.

The numbers that are often included in the names of PEGs indicate their average molecular weights, e.g. a PEG with n=9 would have an average molecular weight of approximately 400 and would be labeled PEG 400. Most PEGs include molecules with a distribution of molecular weights, i.e. they are polydisperse.

PEGs are soluble in water and most organic solvents.