Abstract The oxidation of Si–C–N–O fibres has been investigated. The oxidation rates and the activation energies for the Si–C–O system are similar to those for crystalline SiC. The oxygen and the free carbon concentrations in the ceramics have a limited influence on the oxidation behaviour. As long as the formed silica scale is protective, oxidation kinetics are essentially controlled by the diffusion of oxygen through SiO 2. The parabolic rates in the Si–C–N–O and Si–N–O systems are lower and their activation energies higher than those for SiC. Their values strongly depend on the ratios of C and N bonds to Si and continuously vary from those for SiC ( E a=110−140kJ mol −1) to Si 3N 4 ( E a=330–490 kJ mol −1). The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon-oxynitride (like for Si 3N 4) or silicon-oxycarbonitride layer. The oxidation behaviour of such complex systems is not significantly influenced by the oxygen nor the free carbon contents. It might be governed by the C/Si and N/Si ratios, limiting the nitrogen concentration gradient of the silicon-oxy(carbo)nitride sub-layer and therefore affecting the diffusion/reaction rates.