The oxygen–oxygen bond dissociation enthalpies in di- tert-butyl peroxide (Me 3CO–OCMe 3) and di-trifluoromethyl peroxide (F 3CO–OCF 3) were determined using MP2, the hybrid functional B3LYP, and the CBS-QB3 method. Comparison of the performances of these methods, with particular emphasis on extrapolation procedures to infinite basis-set, showed that only CBS-QB3 is suitable for the accurate calculation of O–O bond dissociation enthalpies. The CBS-QB3 calculations led to DH°(Me 3CO–OCMe 3)=176.8 kJ mol −1and to DH°(F 3CO–OCF 3)=209.4 kJ mol −1. These results were compared with the available experimental data and the prediction for di- tert-butyl peroxide supports literature values that were derived by photoacoustic calorimetry and through a combination of electron affinity and acidity data. The structural analysis of the compounds and their radicals revealed that the lower O–O bond dissociation enthalpy in di- tert-butyl peroxide is mainly related to the stabilization of the tert-butyl peroxyl radical by hyperconjugation.