IsoButane is a branched-chain isomer of normal butane.

Isobutane is particularly valuable in a refinery as a feed stock to the alky unit, where is is combined with either propylene or butylene to make alkylate (a high quality gasoline blend stock).

Isobutane is produced in a variety of conversion units in a refinery. However, when a refinery has an alky there is usually a need to make additional isobutane "on purpose" to keep the alky full. This is done by isomerizing normal butane in a C4 Isomerization unit.