The photoaddition of naphthalene and acrylonitrile has been reinvestigated. It was found that the cyclobutane adduct described previously is endo-7-cyano-2,3-benzobicyclo[4.2.0]-octa-2,4-diene (7), and not the exo-8-cyano compound. The structure was determined from an X-ray diffraction study on the p-bromophenylurethane of the carbinol derived from the cyano-compound. The structural details are described. The crystallographic data are a = 13.85(1), b = 7.976(2), c = 12.752(4) Å, α = 90.80(3), β = 87.02(6), γ = 90.99(5)° space group [Formula: see text], Z = 2 units of C20H18O2NBr. The least-squares residual R was 0.085 for 651 X-ray reflections measured on a Syntex P1 diffractometer using graphite-monochromatized Mo radiation. The methine proton resonances in the 220 MHz n.m.r. spectrum of 7 were resolved using pyridine solvent, and the coupling constants are consistent with the structure and geometry, as derived from the X-ray work. A small fraction (7%) of 8-cyano-2,3-benzo-bicyclo[4.2.0.]octa-2,4-diene was also formed. This is tentatively assigned as the endo isomer, and on base treatment it isomerizes to a new compound, identified as the exo isomer. In contrast, the major cycloadduct 7 is recovered unchanged from base treatment. The folded geometry of the four-membered ring in these adducts apparently determines the stabilities of the epimers, and the proton–proton vicinal coupling constants.