The nanocrystallization process of reactively sputtered thin amorphous Ta–Si–N films is investigated by anomalous small angle x-ray scattering (ASAXS) and x-ray diffraction (XRD). Changes in the microstructure in ${\mathrm{Ta}}_{\mathrm{40}}{\mathrm{Si}}_{\mathrm{14}}{\mathrm{N}}_{\mathrm{46}}$ films, density variations in the amorphous matrix, decomposition, formation, and growth of nanocrystals after vacuum anneals at different temperatures in the range between 800 and 1000 °C are observed and the results of the different techniques are compared. From a Fourier analysis of ASAXS intensities the nanostructure of the investigated ternary system is derived using a model of hard spheres according to Guinier and Fournet. ASAXS investigations indicate that the noncrystalline samples can be described by a monophase fit and the crystallized samples by a bimodal-phase fit, the latter results being consistent with XRD which identifies TaN and ${\mathrm{Ta}}_{\mathrm{5}}{\mathrm{Si}}_{\mathrm{3}}$ phases. Detailed analysis shows that TaN nanograins of approximately 2 nm size develop after a decomposition process. Larger grains of ${\mathrm{Ta}}_{\mathrm{5}}{\mathrm{Si}}_{\mathrm{3}}$ are observed in addition to the TaN grains if annealing is performed at temperatures higher than 950 °C. The aim of these investigations is to give a generally applicable explanation of the barrier failure mechanism for Ta–Si–N diffusion barriers, which is actually observed at temperatures below the crystallization temperature if the films are used in contact with Cu or Al.