The electrochemistry of four redox analytes [Fe(CN)(6)(-3/-4), RU(NH3)(6)(+2/+3), IrCL6-2/-3, and methyl viologen, MV+2/+/0] was investigated at polycrystalline, boron-doped diamond thin-film electrodes before and after anodic polarization and hydrogen plasma treatment. The as-deposited diamond surface is predominantly hydrogen terminated, and quasi-reversible cyclic voltammograms (Delta Ep of 60-80 mV) were observed for all of these couples at 0.1 V/s. After anodic polarization in H2SO4 the surface atomic O/C ratio, as determined by X-ray photoelectron spectroscopy, increased from 0.02 to ca. 0.20. Concomitant with the increase in sur face oxygen, the Delta E-p for Fe(CN)(6)(3/-4) increased to over 200 mV, while the Delta E-p values for the other redox systems remained relatively unchanged. After acid washing and rehydrogenating the surface in a hydrogen plasma (i.e., atomic hydrogen), the Delta E-p for Fe(CN)(6)(3/-4) returned to ca. 80 mV, while the Delta E-p values for the other three redox analytes remained close to the original values. The results demonstrate that electron transfer for ferri/ferrocyanide is very sensitive to the presence of surface carbon-oxygen functionalities and that the electron transfer involves a site associated with the hydrogen-terminated surface. The results also unequivocally rule out the influence of adventitious nondiamond carbon phases as the sole sites for the electron transfer.