Polyethylene glycol (PEG) and polyethylene oxide (PEO) are polymers having an identical structure except for chain length and end groups, and are the most commercially important polyethers. Polyethylene glycol refers to an oligomer or polymer with low molecular weight while polyethylene oxide is used for higher molecular weights. PEG generally is a liquid while PEO is a low-melting solid. Both are prepared by polymerization of ethylene oxide. While they find use in different applications and have different physical properties (i.e. viscosity) due to chain length effects, their chemical properties are nearly identical.

Polyethylene glycol has the following structure:

HO-(CH2-CH2-O)n-H

Pegylation is the act of adding a PEG structure to another larger molecule, for example, a protein (which is then referred to as pegylated).

PEG is soluble in water, methanol, benzene, dichloromethane and is insoluble in diethyl ether and hexane. It is coupled to hydrophobic molecules to produce non-ionic surfactants.