The oxidation behaviour of carbon fibres has been studied in air at different temperatures from 550 to 860-degrees-C. A linear relationship has been observed between the carbon fibre size and oxidation time. Experimental results show that the oxidation process is in a mixed control zone, i.e. controlled both by diffusion of the gaseous reactant and product within the boundary layer and by the chemical reaction. The activation energy of carbon burn-off is 140 +/- 5 kJ mol-1. The uniform nature of the surface oxidation makes it possible to re-size the carbon fibres to smaller diameters.