Electrically conductive nanocrystalline diamond films (approximately 750 to 1000 nm thick) were deposited on conducting Si and W substrates from CH4/N-2/Ar gas mixtures using plasma-enhanced chemical vapor deposition. Such films are continuous over the surface and nanometer smooth. The grain size is 3 to 10 nm, and the grain boundaries are 0.2 to 0.5 nm wide (two carbon atoms). Nitrogen appears to substitutionally insert into the grain boundaries and the film concentration (similar to 10(20) atom/cm(3)) scales with the N-2 added to the source gas mixture up to about the 5% level. The nitrogen-incorporated films are void of pinholes and cracks, and electrically conducting due in part to the high concentration of nitrogen impurities and or the nitrogen-related defects (sp(2) bonding). The films possess semimetallic electronic properties over a potential range from at least -1.5 to 1.0 V vs. SCE. The electrodes, like boron-doped microcrystalline diamond, exhibit a wide working potential window, a low background current, and high degree of electrochemical activity for redox systems such as Fe(CN)(-3/-4)(6), Ru(NH3)(+3/-2)(6), IrCl6-2/-3, and methyl viologen (MV+2/+). More sluggish electrode kinetics are observed for 4-methylcatechol, presumably due to weak adsorption on the surface. Apparent heterogeneous electron transfer rate constants of 10(-2) to 10(-1) cm/s are observed for Fe(CN)(-3/-4)(6), Ru(NH3)(+3/+2) IrCl6-2/-3, and MV+2/+ at films without any pretreatment. (C) 2000 The Electrochemical Society. S0013-4651(00)06-00 1-8. All rights reserved.