Abstract Size controlled silicon nanocrystals (SiNC) in silicon oxynitride matrix were prepared using plasma enhanced chemical vapor deposition. The as-deposited superlattices (SLs) and the corresponding bulk films were treated by thermal annealing. Hydrogen effusion was performed during the heating up by choosing a sufficiently low heating ramp. The phase separation of the layers into SiNCs and surrounding oxynitride matrix was studied at temperatures of up to 1150 °C. The influence of the annealing temperature on SiO xN y/SiO 2 – SLs with varying SiO xN y layer thickness was investigated by several analytical techniques including variable angle spectroscopic ellipsometry, photoluminescence (PL) spectroscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectrometry (FTIR) and transmission electron microscopy (TEM). Before annealing FTIR investigations show in addition to the expected Si-O bonds also the formation of nitrogen and hydrogen related bonds. The shift of the Si-O-Si stretching vibration to higher wave numbers after annealing indicates phase separation. The disappearance of the hydrogen related bonds indicates the hydrogen effusion. The PL signal is rising significantly with increasing annealing temperature and the PL peak position is strongly related to the thickness of the SiO xN y sublayers due to quantum confinement effects. TEM investigations confirm the size-controlled growth of SiNCs within the oxynitride matrix. The role of incorporated nitrogen and hydrogen is discussed.