The interdiffusion of TiN and NbN in superlattice structures has been investigated by studying the evolution of superlattice satellite peaks in x-ray diffraction; as well as by cross-sectional transmission electron microscopy. Single crystal TiN/NbN superlattices with composition modulation periods of 4.4 and 12.3 nm were deposited, by reactive dual-cathode unbalanced magnetron sputtering in an Ar/N-2 discharge, onto MgO(001) substrates held at a temperature of 700 degrees C. Isothermal annealings (in the range of 750-875 degrees C for 20 min) as well as a ramped annealing (3 degrees C s(-1) up to 1200 degrees C) were performed, and in situ x-ray diffraction spectra were continuously recorded using synchrotron light and a linear detector. The results pointed to a nonlinear diffusion in TiN-NbN couples. The structure maintained abrupt interfaces throughout annealing, while the position of the interfaces was continuously shifted into the TiN layers. A model is proposed where Ti diffuses into the NbN layer to form a NbTiN alloy, while the diffusion of Nb in the opposite direction is restricted. Within the temperature range from 750 to 875 degrees C, activation energies for metal interdiffusion were limited to 1.2 eV for the lower temperature end, and 2.5 eV for the higher temperature end. The expected lifetime against alloying has been determined using the random walk theory, and a TiN/NbN superlattice with a period of 4.4 nm is expected to sustain a layered structure for similar to 10 h at 750 degrees C and similar to 2 h at 850 degrees C.