A recently disclosed development(1-3) comprising a composite of flame retardant cellulose fibres and an intumescent has shown unusually high flame and heat resistance with a char structure that can withstand air exposure temperatures up to 1200 degrees C. Char structures of selected flame retardant cellulose fibres when alone and blended with inert E-glass and aromatic char-forming novoloid fibres in the presence of an ammonium polyphosphate-based intumescent have been studied using scanning electron microscopy. Evidence suggests that a ’char-bonded’ structure is formed in which flame retardant cellulose fibrous chars have interacted with intumescent char during its formation. Similar interactions are not seen with E-glass and novoloid fibre residues. The char-bonded structure is considered to have reduced air permeability and so has considerably enhanced surface oxidative resistance at high temperatures. An outline mechanism is proposed and is based on known pyrolysis mechanisms for the flame retarded cellulose fibre and intumescent components.