This article reports on structure, phase composition and high-T oxidation resistance of sputtered Mo-Si-N films. These films were dc reactively sputtered using an unbalanced magnetron equipped with a MoSi2 alloyed target in a mixture Ar and N-2. A continuous increase of partial pressure of nitrogen p(N2) from 0 to 0.6 Pa makes it possible to produce two groups of composites: (1) MoSix +a-Si3N4 and (2) a-Si3N4+MoNx. The composites of the first group are crystalline and contain a low amount of the a-S3N4 phase. On the contrary, the composites of the second group are amorphous and the a-Si3N4 phase dominates in these films. Sputtered films were characterized using XRD, EPMA, microhardness measurements, thermogravimetric measurements and SEM. It was found that the thermal annealing of Mo-Si-N films in flowing air at temperatures T-a>= 900 degrees C results in a loss of the film mass (Delta m < O). This loss of weight is due to the decomposition of MoNx > 1-> Mo+N-(g) and the formation of volatile MOO, oxides, which diffuse out of film. This process results in (i) the formation of thin porous oxide surface layer and (ii) the loss of film mass. A very low (Delta m approximate to 0.01 mg/cm(3)) decrease of the film mass is obtained in the case when the Mo-Si-N film contains a large (> 60 vol %) amount of SO4 phase and stoichiometric (x=1) or substoichiometric (x < 1) MoNx nitride. In these films the loss of weight does not increase with increasing T-a up to 1300 degrees C. This fact demonstrates the high-T oxidation resistance of the a-Si3N4/MoNx < 1 composite. The temperature T-a= 1300 degrees C is not a physical limit of the high-T oxidation resistance of the a-Si3N4/MoNx <= 1 composite but only the limit of Si substrate used in our annealing experiments. The microhardness H of the a-Si3N4/MoNx < 1 composite is also quite high and achieves approximately up to 25 GPa. (c) 2005 American Vacuum Society.