Si-N-(O) fibres were grown according to a high temperature vapour-solid process involving the reaction between SiO and NH3 on a substrate. The oxygen concentration of the fibres is related to the partial pressures of SiO and NH3 during fibre growth, depending respectively, on the processing temperature and the ammonia flow. The fibres consist of amorphous silicon oxynitride of composition SiO2xN4(1-x)/3 (0.1 < x < 0.2). They exhibit a large spread in tensile strength. The lowest values (about 1 GPa) correspond to large surface defects caused by intergrowth while the highest values reach 5 GPa for perfect fibres. The fibres are stable in nitrogen up to 1450 degrees C (10 h) in terms of composition, structure and mechanical behaviour owing to their high processing temperature (1450 degrees C) and the nitrogen pressure preventing decomposition. A superficial crystallization into Si3N4 is only observed at 1500 degrees C inducing a moderate decrease of strength. In argon, decomposition starts at 1400 degrees C yielding gaseous species (SiO and N-2), crystalline Si3N4 and free silicon beyond 1400 degrees C and induce a catastrophic drop of strength. Annealing in oxygen results in a growth of a protective SiO2 scale, amorphous or partially crystalline at 1400 degrees C.