The hydroxyl bond energies in the series of fluorinated and chlorinated methanols CFxH3-xOH and CClxH3-xOH have been examined using ab initio molecular orbital theory. The O-H bond strength is found to increase with the degree of halogenation, with fluorine substituents having a greater effect than chlorine substituents. The bond strengthening is not, however, directly related to the degree of halogenation. Rather, the bond energy appears to increase in a manner that coincides with the effectiveness of negative hyperconjugation between the methyl and hydroxyl groups. The trends are consistent with the notion that bond strengthening arises from a decrease in effectiveness of negative hyperconjugation from the methanol to the methoxy radical, the difference being manifested in a modification of the O-H bond strength. Support for this model is provided by the calculated bond length trends, conformational preferences, radical state energy separations, and orbital populations.