X-ray diffraction measurements and simulations were carried out to characterize the composition modulation and structure of TiN/NbN superlattices. A trapezoidal/sawtooth form of the composition wave was assumed. Random d-spacing fluctuations with Gaussian width approximately 0.002 nm and layer thickness variations of 0.1-0.5 nm were incorporated to match the observed broadening of the peaks. Best fits to the experimental data showed that considerable interdiffusion was present, with up to 15 at.% metal substitution within the layers and interface widths of 0.4-2.0 nm. The interface width values agreed well with those used in hardness enhancement calculations for epitaxial TiN/NbN superlattices.