Boron-doped diamond electrodes, both as-grown and polarized anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the role played by the surface-state density. Many different treatments were employed: as-grown (BDDag), mildly polarized (BDDmild), strongly polarized in perchloric acid (BDDsevere), and strongly polarized in a sulphuric acid-acetic acid mixture (BDDAcOH). Charge transfer processes at the electrode surface were studied by cyclic voltammetry. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe-III/II(CN)(6)) and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H(2)Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modi. cation as a function of surface treatment. The active surface area and the reaction rate constants decreased significantly upon anodic polarization. Important drops in the charge carrier concentration on the surface and in true surface area led to hindrance of electron transfer at the electrode.