We produced nitrogen-and carbon-rich amorphous Si-C-N thin films of chemical composition Si19C21N60 by reactive magnetron co-sputtering on Si substrates and investigated their thermal stability in air at room temperature. Infra-red spectroscopy and X-ray reflectometry measurements demonstrated that the films show a pronounced oxidation in air already at room temperature, a behaviour which was previously not found for other compositions. A linear growth law is observed, which can be explained by a nano-porous structure of the formed SiO2 oxide scale. High-resolution atomic force microscopy indicated the presence of such a nano-porous structure with a pore diameter of 5 nm and a diameter of the pore walls of 3 nm. (C) 2011 Elsevier B.V. All rights reserved.