Epitaxial W/NbN superlattices with modulation wavelengths Delta ranging from 1.3 to 25 nm were grown on MgO(001) substrates by de reactive magnetron sputtering in Ar/N-2 mixtures. The epitaxial relationship between the layers is given by W(001)//NbN(001) and W[110]//NbN[100]. X-ray diffraction and Rutherford backscattering results fitted using simulations showed that the superlattices had well-defined planar layers with interface widths of approximate to0.2 nm. Nanoindentation measurements showed superlattice hardnesses as high as 33 GPa compared to 8 for W and 30 fur NbN. The superlattices showed little change in x-ray superlattice reflections or nanoindentation hardness after vacuum annealing up to the highest temperature tested, 1000 degreesC fur G h. Thus, the layers remained intact during annealing, allowing the superlattice hardness enhancement to be retained.