Solvolysis of 1,3-di-tert-butyl-5-methyl-5-cyclopenta-1,3-dienyl trifluoroacetate (15a) occurs with a strong dependence on solvent ionizing power (m = 0.97), and gives products of substitution, allylic and skeletal rearrangement with substitution, and elimination. These results are characteristic of a process involving an intermediate carbocation, and provide the first measurements of the kinetics of formation of a cyclopentadienyl cation, the prototypical 4 pi-electron carbocation destabilized by antiaromaticity. The reactivity of 15a in 2,2,2-trifluoroethanol at 25 degrees C is calculated to be lower than those of analogous fluorenyl and indenyl derivatives by factors of 3 x 10(4) and 4 x 10(2), and is exceeded by that calculated for 1,3-dimethyl-3-cyclopentenyl trifluoroacetate by a factor of 10(14), showing the large carbocation destabilizing effects of antiaromaticity.