The reaction profile for hydrogen atom transfer from phenol to the methylperoxyl free radical is calculated using B3LYP hybrid density functional calculations. The electronic structure of the transition state reveals partial delocalisation of impaired spin density onto the phenol and electron density removal from the aromatic ring of the phenol. The results provide a clear rationale for the rate enhancing effects of spin delocalising and/or electron releasing substituents on the phenol ring. (C) 2002 Published by Elsevier Science B.V.