Nanocrystalline diamond thin-®lms have been synthesized in CH 4 /Ar (argon) and CH 4 /N 2 gas mixtures without the addition of molecular hydrogen. Atomic force microscopy analysis demonstrates that the C 60 /Ar ®lms consist of hemispherical features about 150 nm in diameter with a height of 20 nm. The mean roughness over a 262 mm 2 area, based on feature height analysis, is 50 + 8 nm. The voltammetric response of these ®lms in the presence of FeCN , methyl viologen and 4-tert-butylcatechol is similar to that expected for high quality microcrystalline diamond. The results indicate that the nanocrystalline ®lms are active without any conventional pretreatment, and possess semimetallic electronic properties over a potential range from 0.5 to À1.5 V (vs. SCE). Nanocrystalline diamond thin-®lms produced from CH 4 /N 2 mixtures are rougher than the C 60 /Ar ®lms, and possess electrochemical properties more closely resembling glassy carbon. The charge carrier concentration in both types of ®lms is believed to result mainly from the sp 2 carbon in the grain boundaries.