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NbC x N 1− x layers were deposited on MgO(001) by reactive magnetron co-sputtering from Nb and graphite targets in 5 mTorr pure N 2 at T s = 600–1000 °C. The anion-to-Nb ratio of 1.09 ± 0.05 is independent of T s and indicates a nearly stoichiometric rock–salt structure Nb(N,C) solid solution. In contrast, the C-to-N ratio increases from 0.20–0.59 for T s = 600–1000 °C, which is attributed to a low C sticking probability at high N surface coverage at low T s . Layers grown at T s ≥ 700 °C are epitaxial single-crystals with a cube-on-cube relationship to the substrate, (001) NbCN ||(001) MgO and [100] NbCN ||[100] MgO , as determined from X-ray diffraction θ –2 θ and ϕ -scans. Reciprocal space mapping on a NbC 0.37 N 0.63 layer deposited at T s = 1000 °C indicates an in-plane compressive strain of − 0.4% and a relaxed lattice constant of 4.409 ± 0.009 Å. The lattice constant of NbC x N 1− x increases with x , consistent with a linear increase predicted by first-principles density functional calculations. The calculated bulk modulus, 307 GPa for NbN and 300 GPa for NbC, is nearly independent of x . Similarly, c 11 increases slightly from 641 to 666 GPa, but c 12 decreases considerably from 140 to 117 GPa, and c 44 more than doubles from 78 to 171 GPa as x increases from 0 to 1, indicating a transition from ductile NbN to brittle NbC. This also results in an increase in the predicted isotropic elastic modulus from 335 to 504 GPa, which is in good agreement with the measured 350 ± 12 GPa for NbC x N 1− x (001) with x = 0.19–0.31. The hardness H = 22 ± 2 GPa of epitaxial NbC x N 1− x layers is nearly independent of x = 0.19–0.37 and T s = 700–1000 °C, but is reduced to H = 18.2 ± 0.8 GPa for the nanocrystalline layer deposited at T s = 600 °C. The electrical resistivity decreases strongly with increasing T s < 800 °C, due to increasing crystalline quality, and is 262 ± 21 μΩ-cm at room temperature and 299 ± 22 μΩ-cm at 77 K for T s ≥ 800 °C, indicating weak carrier localization due to the random distribution of C atoms on anion sites. [ABSTRACT FROM AUTHOR]