Silicon oxynitride films, deposited by chemical vapour deposition using a high He dilution are investigated with respect to their photoluminescence properties. At low temperatures the PL spectra, which show broad maxima around 2 eV, consist of two contributions with different decay times: a ns-fast and a ms-slow component, the slow one disappearing above 80 K. To account for these findings and for absorption measurements, a model is proposed to describe the structure of the samples consisting of inclusions of Si rich regions in an a-SiO
:H matrix with a common molecular aggregate as luminescence center. The different PL lifetimes supposedly result from different feeding mechanisms. Such a behaviour may be present in oxidized porous Si as well.