Hexamethylethane (HME) and trimethylbutane (TMB) radical cations produced in ZSM-5 zeolite by gamma radiolysis undergo an elimination process followed by an ion-molecule reaction. The tetramethylethylene radical cation intermediate formed by CH4 elimination from TMB+. is observed directly in the temperature range 120-200 K. The deprotonation product, (CH3)3CC.(CH3)2, of the cation-molecule reaction is observed for temperatures > 200 K. The isobutene radical cation formed by elimination of C4H10 from HME+. is not observed - presumably because of rapid ion-molecule reaction to form the 2-methylallyl radical, which is observed.