The standard enthalpies of formation of an ubiquinol model (where the isoprenoid units have been changed to a methyl group) and its free radical ubiquinoxy were estimated by DFT computations using isodesmic reactions as working chemical reactions and extended basis sets. In a first step, using aromatic compounds with experimentally well-established enthalpies of formation as test systems, the performance of the DFT functionals, the basis sets, and the working chemical reactions was analyzed. In a second step, the enthalpies of formation of other aromatic species also used as reference compounds in the thermochemical determination of ubiquinol and ubiquinoxy models were updated or calculated for the first time. The values for the 4-hydroxyphenoxy free radical and for 1,2-dimethoxybenzene were -31.6 +/- 2.0 and -47.8 +/- 2.0 kcal mol(-1), respectively, which are reported in this study for the first time. With this information, the recommended values for ubiquinol and ubiquinoxy models are, respectively, -147.0 +/- 2.0 and -118.1 +/- 2.0 kcal mol(-1). The error limits designate the estimated maximum uncertainties. These enthalpies of formation correspond to the O-H bond dissociation enthalpy of BDE(O-H) = 81.0 +/- 2.0 kcal mol(-1), in excellent agreement with the only experimental value for natural ubiquinol, which lends confidence to the method and model used.