The chemical composition, microstructure and mechanical properties of Tyranno Lox-E fibre were studied in the as-received state and after annealing in inert atmosphere. The fibre consists of SiC nanocrystals of 2-3 nm, free carbon aggregates of 4-5 distorted aromatic layers and 1-3 nm in length and an amorphous silicon (titanium) oxycarbide phase. Except for evolution of residual hydrogen and a slight densification, the fibre is chemically and structurally stable and retains a high strength up to 1300 degrees C. Beyond 1300 degrees C, superficial degradation resulting from decomposition of the oxycarbide into SiO(g) and CO(g) induces a decrease of strength. Compared with bulk polycrystalline SiC, the fibre has a low creep resistance at high temperature, mainly because of the nanometric size of the SiC crystals but also because of the presence at the grain boundary of the oxycarbide phase (viscous and chemically unstable) and of the poorly organized free carbon phase (chemically and structurally unstable).