In this work, a polymeric phosphorous-nitrogen containing intumescent flame retardant, named poly(diaminodiphenyl methane spirocyclic pentaerythritol bisphosphonate) (PDSPB), was synthesized. The carbonization chemistry was investigated. FTIR and H-1 NMR were used to confirm the chemical structure of PDSPB. Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), in situ FTIR and energy dispersive X-ray (EDX) were used to investigate and monitor the chemical structural changes during thermal degradation. PDSPB demonstrated a three-step degradation behavior. PDSPB oligomers continuously polymerized and generated a higher macromolecular weight during the first step (200-250 degrees C). The phosphate ester bonds were broken down and phosphoric acid was released which dehydrated the carbon source to form chars during the second step (280-320 degrees C). The residues will be further degraded and form final chars during the final weight loss step (400-450 degrees C). The final chars showed a complex P-O-Ph and aromatic/graphitic structure containing architecture. (C) 2012 Elsevier B.V. All rights reserved.