The relative energies of the three possible isomers of 2,5-dibenzylidenecyclopentanone and of 2,6-dibenzylidenecyclohexanone were calculated using Mechanics, MOPAC, and MOPAC with CI. The calculated lowest energy isomer of each compound agrees with known spectroscopic and crystallographic data. This work shows that the "SCF- CI" calculations previously reported on 2,5-dibenzylidenecyclopentanone do not predict the actual structure of the compound and should not be used to predict the structure of the corresponding polymer.