isobutylenePronunciation: /ˌʌɪsəʊˈbjuːtʌɪliːn, ˌʌɪsəʊˈbjuːtɪliːn/Definition of isobutylene noun [mass noun] Chemistry an easily liquefied hydrocarbon gas used in the making of butyl rubber. Chemical formula: (CH3)2CCH2 isobutylene in other Oxford dictionaries Definition of isobutylene in the US English dictionary