Thin films of nitrogenated amorphous carbon (a-CNx) were deposited on Si(100) substrates by reactive radio frequency sputtering using a gas mixture of Ar and N2 at a total working pressure of 3 mTorr. X-ray photoelectron spectroscopy (XPS) showed that the films consisted of amorphous carbon (a-C) containing a β-C3N4-like phase with N atoms bonded to C atoms in tetrahedral coordination (sp3) and a graphite-like phase with N atoms bonded to C atoms in trigonal coordination (sp2). Analysis of the XPS spectra revealed a strong effect of the N2 partial pressure on the concentration and composition of each constituent. The thickness and nanohardness of the a-CNx films were in the ranges of 7–35 nm and 12.5–40 GPa, respectively, depending on the fraction of N2 in the sputtering gas. Conversely to the chemical composition, the growth rate (thickness), microstructure, and nanomechanical properties of the a-CNx films were found to depend on the total mass flow rate. While the N/C atomic ratio increased with the N2 partial pressure, the film nanohardness and in-plane elastic modulus decreased due to the reduced energetic ion bombardment on the growing film surface and the incorporation of soft phases in the sputtered films. Nanoindentation and XPS results are presented to elucidate the deposition kinetics and to interpret the dependence of the resulting film microstructure and nanomechanical pro- perties on the plasma conditions. © 1999 American Institute of Physics.