Silicon oxynitride thin films, deposited bp r.f. magnetron sputtering, are used to synthesize multilayer stacks such as high reflectivity mirrors. The SiO x N y refractive index and the optical performances of these layers are measured at 1064 nm and the two compositions SiO 2 and Si 3 N 4 were chosen to realize the multilayers. The losses (absorption, scattering) are measured and compared to the performances of the Ta 2 O 5 /SiO 2 mirrors deposited by dual ion beam sputtering. The absorption level (40 ppm) is eight times higher than that of the Ta 2 O 5 /SiO 2 mirrors. This is due to the high absorption coefficient of Si 3 N 4 . In contrast, the scattering is not governed by the material properties but by the deposition process. The r.f. sputtering increases the sample RMS roughness. Solutions are proposed to optimize optical losses of Si 3 N 4 /SiO 2 mirrors.