Ab initio molecular orbital calculations of the structures and energies in the ring opening of cyclobutene-1,2-diones to form 1,3-butadiene-1,4-diones (bisketenes) show that the reaction is endothermic by 3.2 kcal/mol for the hydrogen substituted case, but with a high barrier of 30.8 kcal/mol, at the MP2/6-31G* + ZPVE//MP2/6-31G* level. Substituents SiH3 favor the ring opened bisketene form by 3.9 and 4.4 kcal/mol per group, and lower the barrier by 1.4 and 1.6 kcal/mol per group. Fluoro substituents disfavor the bisketene by 10.3 and 6.7 kcal/mol per group, but one fluorine increases the barrier for ring opening by only 0.8 kcal/mol, while the barrier is 0.9 kcal/mol less for the difluoro derivative than for the parent, indicating that any influence of product destabilization by fluorine on the barrier is compensated by ground state destabilization by fluorine tending to decrease the barrier for ring opening. In all cases twisted conformations are the lowest energy minima for the bisketenes and are favored over the planar forms, which are transition structures for rotation around the single C-C bond. The calculated energies, including enthalpy and entropy changes, are in good agreement with available experimental data. For cyclobutene, cyclobutenone, and cyclobutenedione there is no correspondence between the magnitude of the barriers of the reaction and the overall energy changes, and for cyclobutenone this is attributed partly to a favorable dipolar transition state interaction.