Thermal degradation of cyanoethylated cellulose samples, possessing different degrees of substitution, was investigated using a TGA technique. Pyrolytic degradation of the compounds, except of those of degree of substitution less than 2.0, exhibited three stages. Higher substituted samples passed through four stages. Activation energies, orders of reactions and starting temperatures were estimated for each stage. The introduction of cyanoethyl groups enhances the thermal stability of the cellulosic fibres. Such stability was shown to increase with increase of the degree of substitution. X-Ray diffraction patterns of some selected samples were recorded to show changes in the fibre crystalline structure.