Abstract: Ab initio and DFT quantum chemical calculations have been applied to a study of Diels-Alder reactions of o-benzoquinone as diene and norbornadiene as dienophile. Transition states for these reactions are located and activation energies estimated. The prefered exo-¶- facial selectivities and exo,endo- stereospecificities exhibited in these cycloadditions are readily predicted using RHF/3-21G or higher levels of calculations. Differences between experimentally observed results and calculations may be explained by the postulation of a second, nonconcerted biradical mechanism leading to formation of hetero Diels-Alder products.