The geometries of bicyclo[2.2.1 ]hepta-2,5-dien-7-one (1) and of the concerted, synchronous transition state for formation of benzene and carbon monoxide from 1 were obtained from ab initio calculations. A nonsynchronous transition state was not found at the UHF/4-31G level, despite an extensive search. At the MP3/6-31G* level, the calculated activation energy for the decarbonylation is in agreement with experiment. Calculations for synchronous transition states reproduce the experimental correlation between the activation energy and the heat of reaction for a series of allowed thermal reactions. Tentative assignments of the observed IR spectrum for 1 and 1-d6are made based on the RHF/3-21G vibrational frequencies and intensities. The factors contributing to the facile decarbonylation are considered to be the availability of an allowed pathway for the reaction and the exothermicity of the reaction, due to the ring strain of the reactant and the stability of the products.