a colourless, volatile liquid hydrocarbon obtained in processing petroleum or coal tar and used as a chemical raw material. The formula is C₅H₈.

Isoprene, either alone or in combination with other unsaturated compounds (those containing double and triple bonds), is used principally to make polymeric materials (giant molecules consisting of many small, similar molecules bonded together) with properties dependent upon the proportions of the ingredients as well as the initiator (substance that starts the polymerizing reaction) employed. The polymerization of isoprene using Ziegler catalysts yields synthetic rubber that closely resembles the natural product. Butyl rubber, made from isobutene with a small amount of isoprene, using aluminum chloride initiator, has outstanding impermeability to gases and is used in inner tubes.

Many plant substances have formulas that are small multiples of C₅H₈. The formation of isoprene upon thermal decomposition of these materials led to the proposal by the German chemist Otto Wallach (Wallach, Otto) in 1887 that they are built up from isoprene units. This “isoprene rule” was verified in numerous cases and has proved useful in studies of the structures of terpenes and terpenoids.