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Abstract: MoN x O y films were deposited on steel substrates by dc reactive magnetron sputtering. The depositions were carried out from a pure molybdenum target, varying the flow rate of reactive gases. X-ray diffraction (XRD) results revealed the occurrence of cubic MoN x and hexagonal (δ-MoN) phases for the films with high nitrogen flow rates. The increase of oxygen content induces the decrease of the grain size of the molybdenum nitride crystallites. The thermal stability of a set of samples was studied in vacuum, for an annealing time of 1h, for temperatures ranging from 500 to 800°C in 100°C steps. The results showed that pure molybdenum nitride films changed their structure from a meta-stable cubic MoN to hexagonal δ-MoN and cubic γ-Mo2N-type structures with increasing annealing temperatures. The samples with molybdenum nitride films evidenced a good thermal stability, but those with molybdenum oxynitride coatings showed a tendency to detach with the increase of the annealing temperature. [Copyright &y& Elsevier]