The physical and electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes, prepared with varying levels of sp(2)-bonded nondiamond carbon impurity, were systematically investigated. This impurity was introduced through adjustment of the methane-to-hydrogen (C/H) source gas ratio used for the deposition. Volumetric gas ratios of 0.3, 0.5, 1, 2, 3, and 5% were employed. Proportional increases in the fraction of grain boundary, the extent of secondary nucleation, and the sp(2)-bonded carbon impurity content resulted in increasing C/H ratio. Variations in the morphology and microstructure were monitored using atomic force microscopy (AFM) and Raman spectroscopy, respectively. The electrode response was assessed using Fe(CN)(6)(3-/4-), Ru(NH3)(6)(3+/2+), Fe3+/2+, and 4-tert-butylcatechol (4-tBC). All were 1 mM in concentration and dissolved in either 1 M KCl or 0.1 M HClO4. While increased sp(2)-bonded carbon content had little effect on the cyclic voltammetric peak separation (DeltaE(p)) and peak current for the first two redox systems, the impurity had a significant impact on the latter two, as DEp decreased proportionally with increased sp(2)-bonded carbon content. The effect of the impurity on the reduction of oxygen in 0.1 M HClO4 and 0.1 M NaOH was also investigated. A direct correlation was found between the relative amount of the impurity, as estimated from Raman spectroscopy, and the overpotential for oxygen reduction. The greater the nondiamond content, the lower the kinetic overpotential for the reduction reaction. Tafel plots yielded an apparent exchange current density that increased and a transfer coefficient that decreased with the increased nondiamond carbon content. The results demonstrate that the grain boundaries, and the sp(2) carbon impurity presumably residing there, can have a significant impact on the electrode reaction kinetics for certain redox systems and little influence for others. (C) 2004 The Electrochemical Society.