This work studies the thermooxidative degradation of cotton and modacrylic fabrics and of a cotton-modacrylic blend. The association in a blend of modacrylic with cotton fibres is used to improve the fire behaviour of the latter. Using the IKP method which allows the degradation of the material to be modelled, we demonstrate that better thermal stability does not imply better fire-proofing properties, but that these are controlled by the fuel flow rate feeding the flame.