The specific decomposition rates of chemically activated methylcyclobutane produced from CH2(1A1) reaction with cyclobutane have been determined. CH2(1A1)was produced from ketene photolyses at 3340 and 3130 Å and from diazomethane photolyses at 4358 and 3660 Å. Comparisons of the excitation energies of the methylcyclobutane, determined by RRKM theory calculations, and the experimental results for the ketene systems, with thermochemically predicted maximum excitation energies, favor an Arrhenius A factor in the range of 5 × 1015 to 1 × 1016 sec−1 for methylcyclobutane. This result is consistent with (1) the comparison of RRKM theory calculations and the experimental unimolecular falloff for methylcyclobutane, (2) the comparison of experimental A factors for cyclobutane and other alkylcyclobutane decompositions, and (3) two out of three reported experimental A factors for methylcyclobutane. An analysis of these and previous results leads to a value of the CH2(1A1) ↔ CH2(3B1) energy splitting of 9±3 kcal/mole.