Zinc nitride films are known to readily oxidize in an ambient atmosphere, forming a ZnO/Zn(OH)2 medium. We report that post-growth thermal annealing significantly improves the stability of zinc nitride with a three-order magnitude increase in degradation time from a few days in un-annealed films to several years after annealing. A degradation study was performed on samples annealed under a flow of nitrogen at 200–400 °C, which showed that the stability of the films depends strongly on the annealing temperature. We propose a mechanism for this improvement, which involves a stabilization of the native oxide layer that forms on the surface of zinc nitride films after exposure to ambient conditions. The result holds significant promise for the use of zinc nitride in devices where operational stability is a critical factor in applications.The study reported in this paper was funded by the EPSRC (Fund code EP/M507611/1) and Johson Matthey PLC. The financial support by these parties is highly appreciated.The data required to reproduce each figure panel is provided in different text files. Columns are separated by a space character. The first line in each column describes the dataset. The complete figures are also included as .tif files.With the added context of the paper, it should be easy to reproduce the figures.Figure 1: Transmittance spectra of (a) an as-deposited and (b) an annealed 700 nm Zn$_{3}$N$_{2}$ film for up to 18 weeks. (c) Scanning Electron Microscopy image of a partially oxidised Zn$_{3}$N$_{2}$ film.Figure 2: (a) Refractive index, n, and (b) extinction coefficient, k, of the Zn$_{3}$n$_{2}$ layer obtained by spectroscopic ellipsometry for different annealing temperatures.Figure 3: Thickness of (a) the Zn$_{3}$N$_{2}$ layer and (b) oxide layers over several months for annealed samples. (c) Lifetime of the Zn$_{3}$N$_{2}$ layer as a function of annealing temperature. The data points marked with * were extrapolated based on the oxidation rates measured in this study.Figure 4: X-ray diffraction scans of the (400) peak for Zn$_{3}$N$_{2}$ samples annealed at different temperatures. The dashed line shows the expected position of the (400) peak based on crystallographic data.