The concept of non-diamond sp2 impurity states as charge transfer mediators on boron-doped diamond (BDD) surface was suggested as an explanation for the electrochem. behavior of synthetic diamond-based electrodes. To verify this concept, graphite particles (sp2) were deposited on diamond electrodes (sp3) by mech. abrasion. The behavior of the so prepd. diamond-graphite composite electrodes were compared with those of as-grown (BDDag) and those after mild anodic polarization (BDDmild). Outer-sphere electron transfer processes such as ferri/ferrocyanide (Fe(CN)6III/II) and inner-sphere charge transfer reactions such as 1,4-benzoquinone/hydroquinone (Q/H2Q) were chosen to investigate the electrochem. properties of these composite electrodes. Both redox systems became more reversible as the graphite (sp2) loading increased. A strong analogy existed between as-grown diamond electrodes and diamond-graphite composite electrodes. Finally a model is proposed which describes the BDD electrode surface as a diamond matrix in which non-diamond (sp2) impurity states are dispersed. These non-diamond sp2 states on BDD surface acts as charge mediators for both inner-sphere and outer-sphere reactions. [on SciFinder (R)]