Propylene oxide is an epoxide with the molecular formula C₃H₆O. This colourless volatile liquid with an odour similar to ether, is produced on a large scale industrially. Its major application is its use for the production of polyether polyols for use in making polyurethane plastics. It is a chiral epoxide, although it is commonly used as a racemic mixture.

This compound is sometimes called 1,2-propylene oxide to distinguish it from its isomer 1,3-propylene oxide, better known as oxetane.

Industrial production of propylene oxide starts from propylene. Two general approaches are employed, one involving chlorohydrin formation and the other involving oxidation. In 2005, about half of the world production was through chlorohydrin technology and one half via oxidation routes. The latter approach is growing in importance.

The traditional route proceeds via the conversion of propylene to propylene chlorohydrin according to the following simplified scheme:

The mixture of 1-chloro-2-propanol and 2-chloro-1-propanol then undergoes internal cyclization. For example:

Lime (calcium hydroxide) is often used to absorb the HCl.

The other general route to propylene oxide involves oxidation of propylene with an organic peroxide. The reaction follows this stoichiometry:

The process is practiced with four hydroperoxides: