Catalytic decomposition behaviour polypropylene (PP) is studied over nanocrystalline HZSM-5 (n-HZSM-5) catalyst. The catalyst is synthesized using the standard procedure reported in the literature. The catalyst is characterized by X-ray diffraction, scanning electron microscopy and nitrogen adsorption studies. Catalytic decomposition of PP is studied at five different heating rates. Vyazovkin model-free kinetic analysis is carried out to understand the variation of activation energy, E-alpha with conversion, alpha. Result shows that n-HZSM-5 catalyst has excellent catalytic effect in terms of reduction in activation energy due to its high external surface area (364 m(2) g(-1)) due to its nanometric dimensions. Presence of the catalyst significantly reduces the maximum decomposition temperature, T-m of the PP sample and the optimum catalyst percentage is around 50 wt%, where the reduction in T-m is around 161 degrees C. Constant pattern behaviour in the thermogravimetric (TG) and derivative thermogravimetric (DTG) indicates existence of similar reaction mechanism. Model-free kinetic analysis indicates that E-alpha decreases slowly and continuously with alpha after alpha >= 0.1. The microporous and mesoporous catalytic activities of the different catalysts control the variation of E-alpha with alpha at higher values of alpha. (c) 2007 Elsevier B.V. All rights reserved.