The characteristics of thermooxidative degradation of fibres in the presence of flame retardants, manifested by predominance of cross-linking processes which favors formation of CR over degradation processes, are established. Modification with flame retardants decreases the yield and toxicity of combustible volatile products, the rate of thermolysis, the activation energy of dehydration (for VF) and cyclization (for PAN fibres), and the exothermic effects and reverse heat flow to the polymer. The fibres obtained have OI of 32 vol. % and are resistant to wet treatments with a fireproofing effect.