Ethylene oxide (C₂H₄O) is an example of a recently identified interstellar species. Unlike most known astromolecules, it has a ring structure. In fact, three-membered rings are rare even in terrestrial chemistry due to the amount of strain involved. Ethylene oxide (or EtO) is an isomer of two more stable molecules, acetaldehyde (CH₃CHO) and vinyl alcohol (CH₂CHOH), another recently identified interstellar species. EtO is an epoxide similar to other ethers but particularly reactive.

Like many astromolecules, EtO was identified in the interstellar medium by means of its characteristic rotational spectrum, which gives rise to transitions in the microwave region of the electromagnetic spectrum.