Reaction rates for the structural isomerization of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-pentene have been measured over a wide temperature range, 672-750 K in a static reactor and 1000-1120 K in a single-pulse shock tube. The combined data from the two temperature regions give Arrhenius parameters E a = 64.7 (±0.5) kcal/mol and log 10 (A, s −1 ) = 15.47 (±0.13). These values lie at the upper end of the ranges of E a and log A values (62.2-64.7 kcal/mol and 14.82-15.55, respectively) obtained from three previous experimental studies, each of which covered a narrower temperature range. The previously noted trend toward lower E a values for structural isomerization of methylcyclopropanes as methyl substitution increases extends only through the dimethylcyclopropanes (1,1-and 1,2-); E a then appears to increase with further methyl substitution. In contrast, the pre-exponential factors for isomerization of cyclopropane and all of the methylcyclopropanes through tetramethylcyclopropane lie within ±0.3 of log 10 (A, s −1 ) = 15.2 and show no particular trend with increasing substitution. C 2006