The thermal behaviour of cellulose and cellulose diisoamylaminophosphate (CDAP) samples having different degrees of substitution have been studied using, DTA and TG techniques from ambient temperature to 700degreesC in static air. The various reactions involved, namely, dehydration, dephosphorylation, dehydrohalogenation, oxidative decomposition and oxidation of charred residue, have been discussed. Brodio method is used to obtain various kinetic and thermodynamic parameters from TG curves for different stages of thermal degradation. The values of activation energies for the oxidative decomposition of CDAP samples are found to be in the range of 49.02-64.19 kJmol(-1) and are lower than that of cellulose (160.74 kJmol(-1)). The IR spectra of the pyrolysis residues of CDAP indicate formation of compounds containing C=O, C=C and P=O groups. The EPR signals indicate that the free radical mechanism is not dominant in the degradation of CDAP. Higher char yields of CDAP samples as compared to cellulose along with related observations suggest that CDAP is an effective flame retardant. Scanning electron micrographs of CDAP show that the fibrillar reaction sites of cellulose have mostly reacted, although the general morphology of the original fibre remains intact. Internal axial cracking and the increased surface area in the chars of treated samples show the intumescent nature of CDAR The mechanism of thermal degradation of CDAP has been proposed.