We report herein the results of a high-level ab initio study of the D-2d and C-2h structures of 1,3,5,7-cyclooctatetraene, GOT, and bicyclo[4.2.0]octa-2,4,7-triene, BOT. This information has been used to obtain, seemingly for the first time, the standard heat of formation of COT in the gas phase. Values obtained by means of standard atomization reaction are higher than the experimental value (4.7 kcal mol(-1) at the G2 level). When isodesmic or homodesmotic reactions are used, the computational and experimental values agree well within the experimental uncertainties.