Single-layered zirconium oxynitride (ZrNxOy) thin films have been deposited on steel substrates, at a constant temperature of 300 °C, by radiofrequency (rf) reactive magnetron sputtering of a pure Zr target in an argon–oxygen–nitrogen atmosphere. The variation of the flow rate of the reactive gases enabled changes in the composition and structure of the films. X-ray diffraction (XRD) and glancing incidence X-ray diffraction (GIXRD) were used to study the as-deposited films and their structural changes during or after heat treatment, from 400 to 900 °C, in controlled atmosphere and in vacuum.The as-deposited films revealed the occurrence of a face-centred cubic (fcc) phase (Zr–N type), but a Zr–N oxygen-doped phase (Zr–N–O) may be also present depending on the oxygen content in the films. Heat treatment above 600 °C reveals the appearance of a tetragonal phase of zirconium oxide.The results are discussed as a function of the chemical composition of the films, annealing temperature, and type of the annealing process.