Perfluoroisobutene (PFIB) is the perfluorocarbon counterpart of the hydrocarbon isobutene and has the formula (CF3)2C=CF2.

A fluoroalkene, it is a colorless gas that is notable as a highly toxic perfluoroalkene.

[1] It is a Schedule 2 substance of the Chemical Weapons Convention.

It forms addition compounds with thiols, and it is this reactivity that may be related to its toxicity.

[1] PFIB is a product of pyrolysis of polytetrafluoroethylene (PTFE), one of the substances invoked to explain polymer fume fever.