The evolution of the chemical composition of the outer layer of SiC-based fibres was studied under different conditions of temperature and gas environment. After heating the fibres under vacuum or in argon at 1270, 1470, and 1570 K, their reactivity was determined with molecular oxygen at 1170 K and their surface composition was analysed by X-ray photoelectron spectroscopy. Treatment under a vacuum led to severe degradation of the fibres due to chemical reactions between SiC, SiO2 and carbon. As a result, the silica layer initially present at the surface of the fibre disappeared and an oxygen-deficient silica compound was formed. On the contrary, the formation of silica at the fibre surface was observed during the treatment in argon and the chemical changes were more limited.