The dependence of rate, entropy of activation, and (¹H/²H) kinetic isotope effect for H-atom transfer from a series of p-substituted phenols to cubane Mn₄O₄L₆ (L = O₂PPh₂) (1) reveals the activation energy to form the transition state is proportional to the phenolic O–H bond dissociation energy. New implications for water oxidation and charge recombination in photosystem II are described.