The nucleation of epitaxial transition-metal nitride films with different lattice mismatch and growth of the corresponding supcrlattices were studied. In-situ Auger electron spectroscopy (AES) was used to characterize the nucleation mechanisms of V0.3Nb0.7N films on TiN (1.7% mismatch), TiN films on VN (2.4% mismatch), TiN films on NbN (3.6% mismatch), NbN films on VN (5.7% mismatch), and VN films on NbN grown by ultra-high vacuum reactive magnetron sputtering. The initial growth mechanism was layer-by-layer for mismatch values up to 3.6%, but three-dimensional island nucleation occurred for 5.7% mismatch. Scanning tunneling microscopy (STM) observations of NbN films showed threedimensional islands, confirming the AES results. X-ray diffraction spectra from the corresponding superlattices showed strong superlattice reflections except for the large-mismatch NbN/VN combination. Cross-sctional transmission electron microscopy studies showed that the superlattice layers were.velldefined and planar for low mismatch, but were irregular and non-planar for VN/NbN, in agreement with the X-ray, AES, and STM results. The maximum lattice mismatch for epitaxial nitride superlattice growth was thus limited by island nucleation and in the range from 3.6 - 5.7%.