Ethyne, or acetylene, has the chemical formula CH₂CH₂, with a triple bond between the two carbons. It is the simplest member of the alkynes. At standard temperature and pressure, ethyne is a colorless gas with a pleasant smell. It is virtually insoluble in water. Both the gas and liquid forms of ethyne are very explosive. Ethyne burns with a very sooty flame, showing its high percentage of carbon content.

Traditionally, the manufacture of ethyne has been by the controlled action of water on calcium carbide, and this reaction has been used to provide the acetylene that is burned by gas miners' and cavers' lamps. This method of production is still used although oxidation of methane, pyrolysis of alkanes, and cracking of hydrocarbons are more commonly encountered. Ethyne essentially does not naturally occur.

Ethyne is mixed with oxygen to produce a hot flame in oxy acetylene welding. It is also used as a starting point in the manufacture of many organic compounds. Poly vinyl chloride (PVC), various other plastics, and ethanal (acetaldehyde) are the most important.

In common with other unsaturated hydrocarbons, the triple bond can be broken down with the addition of hydrogen, or a number of metals, to produce the appropriate metal salt. The properties of ethyne are typical of the alkynes, and very similar to those of the alkenes due to the unsaturated nature of the bond between the carbons.