New ab initio computational efforts to understand the thermal stereomutation reactions shown by isotopically labeled cyclopropanes have provided energies, geometries, and harmonic vibrational frequencies for cyclopropane and for five trimethylene diradical stationary points : one second-order stationary point, one intermediate, and three transition structures. The calculated vibrational frequencies for cyclopropane and the three trimethylene transition structures, and for the various cyclopropane-1,2-d(2) and -1,2,3-d(3) and the 43 distinct trimethylene transition structures derived from them through carbon-carbon bond cleavages, led to calculated kinetic isotope effect values for the elementary reactions taking place by way of these transition structures. A kinetic model for net one-center and two-center thermal epimerizations utilizing the relative transition structure energies obtained from the ab initio computations and the calculated k(H)/k(D) effects on elementary reactions gave a predicted ratio k(1)/k(12) of 1.06/1 for cyclopropane. Deuterium-labeled cyclopropanes are predicted to behave similarly. The net one-center versus two-center thermal epimerization balance in these cyclopropanes results from the participation of multiple kinetically significant reaction paths.