The strength of the O-O bond is of fundamental importance in a variety of chemical processes. Traditionally, a value of 34 kcal/mol has been ascribed to a generic O-O bond dissociation energy. The present, high-level ab initio calculations indicate that the average O-O bond energy is significantly higher, ca. 45 kcal/mol, and that the bond energy is sensitive to the bonding environment. Calculations at the G2 level of theory give bend dissociation enthalpies at 298 K of 50 kcal/mol for HOOH, 45 kcal/mol for CH3OOH, 39 kcal/mol for CH3OOCH3, and 48 kcal/mol for HC(O)OOH and CH3C(O)OOH. The G2(MP2) results are similar and, additionally, give bond dissociation enthalpies of 38 kcal/mol for diacetyl peroxide, 49 kcal/mol for trifluoroperoxyacetic acid, 23 kcal/mol for isopropenyl hydroperoxide, and 22 kcal/mol for peroxynitrous acid.