Flame retardants when applied to cellulosic fibres, change their decomposition in such a way that significant conversion to carbonaceous char occurs. Intumescents, when dispersed on these flame-retardant (FR) fibres, enhance this property. They not only produce an expanded and thermally protective char barrier, but also interact with FR fibres to form a so-called char-bonded structure which has been shown to possess unusually high resistance to air oxidation at temperatures in excess of 500 degrees C. The effectiveness of the char-bonded structure as a flame and heat barrier is considered to be dependent on the efficiency of the interaction of fibre and intumescent char-forming chemistries. Previous studies have demonstrated the interaction between various flame-retarded viscose fibres and phosphate-based intumescents; current research extends the work to include flame-retardant cottons. In this paper, these interactive properties are studied by using the thermal analytical techniques, TMA, TGA and DSC, which enable the various pyrolysis transitions and associated volatilisation and intumescent char-formations to be studied. In this way a greater understanding of the mechanism of complex char-formation and its subsequent oxidation may be investigated.