The kinetics of the metastable dissociation of energy-selected 1,3-butadiene and 3-methylcyclopropene cations to form C3H3+ (cyclopropenyl cation) and CH3 have been investigated by threshold photoelectron photoion coincidence (TPEPICO) time-of-flight mass spectrometry, ab initio molecular orbital calculations, and RRKM statistical theory. Both the experimental results and the molecular orbital calculations indicate that 1,3-butadiene ions lose CH3 by first isomerizing to a higher energy structure (3-methylcyclopropene cation) which can rapidly lose CH3 or isomerize back to the 1,3-butadiene cation. A complete kinetic model of the two-well potential and its three rate constants is necessary to account for the measured dissociation rates as a function of the ion internal energy. At low energies, the dissociation rate is limited by the bond cleavage step, while at higher energies, the bottleneck shifts to the lower energy isomerization step. a fit of calculated RRKM rate constants to the experimental data yields a 0 K isomerization barrier (relative to the 1,3-butadiene ion) of 2.02 +/- 0.03 eV and an activation entropy at 600 K of -4 cal K-1 mol(-1). The entropy of activation for the dissociation step from 3-methylcyclopropene was found to be +7 cal K-1 mol(-1). 3-Methylcyclopropene was found to have an adiabatic ionization energy of 9.28 +/- 0.05 eV and a neutral heat of formation of 273 +/- 2 kJ mol(-1) at 0 K (257 +/- 2 kJ mol(-1) at 298 K). This is the first experimental determination of this value.