The effect of thermal treatment on the properties and structure of carbon-silicon alloy fibres produced from a novel silicon-containing carbon precursor is reported. The precursor, containing about 22 wt % Si, was melt spun into fibres and then oxidatively stabilized under different conditions to render the fibres infusible. The fibres were pyrolysed and heat treated to 1600 degrees C in inert atmosphere. The extent of stabilization was found to be critical to the development of mechanical strength of the fibres which varied with heat treatment temperature, showing a maximum at 1200 degrees C when the strength was 1.2-1.4 GPa. Moduli were low because of the lack of orientation of the carbon layer planes along the fibre axis. The maximum strength and the thermal stability at high temperatures is considerably reduced if the fibres are excessively oxidized at the stabilization stage. Optimally stabilized fibres show a drop in strength at 1300 degrees C but this stabilizes at about 600 MPa over the range 1300-1600 degrees C. These strengths are remarkably good considering the low modulus which is due to the quite high failure strains. The fibres can show excellent resistance to oxidation if given an initial short exposure to oxygen at high temperature. This is considered to be due to an imperceptible layer of silica.