Under thermodynamically well defined conditions, "reducing", "neutral" and "oxidizing" atmospheres, a two-dimensional SiC-C-SiC(O)(fibre) composite was annealed at different temperatures and times. The microstructural evolution has been studied from a macroscopical scale down to the atomic level by scanning election microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), (EELS) and HREM. Corrosion models have been developed; in all atmospheres the corrosion scale grows from the surface into the bulk, giving rise to a very irregular scale due to coarse macroporosity. At high temperature, the fibre degrades by decomposition of its oxycarbide phase a nd by Ostwald ripening of the silicon carbide and graphite crystals inside the fibre, in oxidizing and neutral atmospheres a silica surface scale forms. In oxidizing and neutral atmospheres, the carbon interphase is gradually substituted by a silica layer or mixed silica/silicon carbide layer, respectively, with increasing thickness. In reducing atmosphere, the degradation is the most important since no protective layer forms around the fibre. The carbon interphase reacts with the oxycarbide of the fibre and forms gaseous products and silicon carbide, which replaces the carbon interphase.