The Arrhenius parameters for the gas phase, unimolecular structural isomerizations of 1,1,2-trimethylcyclopropane to three isomeric methylpentenes and two dimethyl-butenes have been determined over a wide range of temperatures, 688-1124 K, using both static and shock tube reactors. For the overall loss of reactant. E a =63.7 (±0.5) kcal/mol and log 10 A= 15.28 (±0.12). These values are higher by 2.6 kcal/mol and 0.7-0.8 than previously reported from experimental work or predicted from thermochemical calculations. E a for the formation of trans-4-methyl-2-pentene is 1.5 kcal/mol higher than E a for the formation of the cis isomer, which is identical to the E a difference previously reported for the formation of trans- and cis-2-butene from methylcyclopropane. Substitution of methyl groups for hydrogen atoms on the cyclopropane ring is expected to weaken the C - C ring bonds, and it has been reported previously that activation energies for structural isomerizations of methylcyclo-propanes do decrease substantially over the series cyclopropane > methylcyclopropane > 1, 1-or 1,2-dimethylcyclopropane. However, the present study shows that the trend does not continue beyond dimethylcyclopropane isomerization. Besides reductions in C - C bond energy, steric interactions may be increasingly important in determining the energy surface and conformational restrictions near the transition state in isomerizations of the more highly substituted methylcyclopropanes. © 2006 Wiley Periodicals, Inc.