SCF, MCSCF, and CI calculations have been carried out to study thermal interconversions occurring on the singlet C3H4 surface. Transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclopropylidene were determined. In addition, the zero-point energies were calculated, and the activation energies for pertinent reactions were evaluated. The thermal rearrangement of allene to methylacetylene was found to proceed in four steps via vinylmethylene, cyclopropene, and propenylidene with the activation energy of 65.8 kcal/mol, which is in good agreement with the observed values of 60.5 and 63.8 kcal/mol. The same reaction paths can also apply to pyrolysis of cyclopropene, in which it undergoes conversion to methylacetylene via propenylidene more easily than to allene via vinylmethylene; the calculated activation energies are 38.1 and 43.4 kcal/mol, respectively. These are again in excellent agreement with the observed values of 37.5 and 43.3 kcal/mol. The activation energies for the allene to cyclopropylidene and the reverse conversions were calculated to be 72.2 and 10.2 kcal/mol. This indicates that cyclopropylidene may not be involved in the interconversion of allene, cyclopropene, and methylacetylene. One of the significant findings in this study is the reaction path for the cyclopropene to methylacetylene conversion via propenylidene, which is less energy demanding than that via vinylmethylene. This made the calculated mechanisms in accord with the experimental data. Furthermore, we will present and discuss reaction mechanisms for pyrolysis of singly and doubly substituted cyclopropene in which this particular reaction path is expected to play a dominant role. © 1989, American Chemical Society. All rights reserved.