The exchange reaction betweent-butyl cation and isobutane in sulfur dioxide solution is first order in both species withEa = 3.6 kcal/mole and ΔS*298 = −27 e.u. No reaction is observed, under these conditions, between thet-butyl cation andn-butane or neopentane. In a similar system the isopropyl chloride – arsenic pentafluoride complex reacts rapidly withn-butane to yield propane and thet-butyl cation, but does not react with neopentane or cyclopropane. An exchange reaction was also observed between the isopropyl chloride – arsenic pentafluoride complex and propane.