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The simplest member of the olefinic hydrocarbon series and one of the most important raw materials of the organic chemical industry; chemical formula, CH2 {double bond} CH2. It occurs in both petroleum and natural gas, but the bulk of the industrial material is produced by heating of higher hydrocarbons. Numerous large-scale processes have been developed, using raw materials ranging from ethane to whole crude oil; in the U.S., ethane and ethane-propane mixtures are commonly used.

Ethylene undergoes polymerization (combination of small molecules to form large molecules) to polyethylene, a plastic material having many uses, particularly in packaging films, wire coatings, and squeeze bottles. The polymerization may be carried out at high pressures and temperatures or by the more recently introduced Ziegler process, which uses a catalyst. With sulfuric acid, ethylene forms a mixture of sulfates that can be hydrolyzed to ethyl alcohol; it combines with chlorine or bromine to yield the corresponding ethylene dihalides, useful solvents and gasoline additives. The reaction of ethylene and oxygen gives ethylene oxide, used in the manufacture of antifreeze, detergents, and other derivatives. Ethylene and benzene combine to form ethylbenzene, which is dehydrogenated to styrene for use in the production of plastics and synthetic rubber.

In botany, ethylene is a plant hormone that inhibits growth and promotes leaf fall. In fruit, how-ever, ethylene is regarded as a ripening hormone. Involved in its action in fruit is some other
factor that influences ethylene sensitivity of the tissues.

Pure ethylene is a colourless, flammable gas having a sweetish taste and odour; it freezes at -169.4° C (-272.9° F) and boils at -103.9° C (-155.0° F).