The electrochemical behavior of highly conductive, boron-doped polycrystalline diamond thin films for oxygen reduction was examined in 0.5 M H2SO4 using linear sweep voltammetry. When the potential sweep is confined to the region negative of 0.0 V vs. Ag/AgCl, oxygen reduction is highly inhibited with cathodic current being observed at similar to -0.6 V vs. Ag/AgCl, as compared with the standard potential for the two-electron reduction of oxygen (O-2 + 2H(+) + 2e(-) = H2O2, E degrees = 0.47 V vs. Ag/AgCl at pH 0). The extreme inhibition of oxygen reduction may be due to an absence of catalytic sites. When the potential is swept to potentials positive of +1.4 V vs. Ag/AgCl, the subsequent sweep into the negative region shows a reduction peak due to oxygen reduction. In this case, catalytic oxygen-containing functional groups can be formed on spt carbon impurities. Relatively mild conditions are required to deactivate me catalytic functional groups, but strong oxidative treatment in base appears to substantially remove the sp(2) carbon impurities. The oxygen reduction behavior in acid solution could be useful in characterizing diamond electrodes, i.e., as a diagnostic for me presence of sp(2)-type carbon on chemical-vapor-deposited diamond thin-film electrodes. It is proposed that diamond electrode surfaces free of sp(2) carbon are highly insensitive to oxygen, which could be a useful feature in electroanalysis.