Nitrogen has been successfully incorporated into nanocrystalline diamond films produced by a CH4/N2 microwave plasma-enhanced chemical vapor deposition method. High mass resolution secondary ion mass spectrometry (SIMS) characterization shows that the density of the incorporated nitrogen, monitored via CN-, can be as high as 1021 atoms/cm3, depending on the ratio of CH4 to N2 in the reactant gas and on the substrate temperature used for the film preparation. SIMS depth profiles demonstrate that the incorporated nitrogen is uniform within the diamond films (about 1.5 μm thick) except at the surfaces and at the interface between film and substrate. Furthermore, the SIMS analyses reveal that alkali elements such as Na, K, and Li appear to be contaminants at the surface of nanocrystalline diamond films produced using a CH4/Ar or CH4/H2 discharge, but are bulk impurities only for the films prepared using a CH4/N2 plasma. These alkali impurities can play an important role in electronic properties such as electron field emissions of nanocrystalline diamond thin films. © 1999 American Vacuum Society.