The deactivation behaviour of the HZSM-5 zeolite in the catalytic cracking of LDPE has been analysed. Four important properties of the partly deactivated catalysts have been measured: firstly, the activity of the fresh and partly coked zeolites in the pyrolysis reactions in TG; secondly, the amount of coke deposited on the catalysts after each reaction; thirdly the effect of the coke content of the catalyst on their acid properties and in the fourth place the effect of the coke content on their surface properties. The experimental study of the deactivation behaviour of HZSM-5 during the catalytic cracking of LDPE has permitted a new and simple kinetic model to be developed that takes into account the effect of the coke formation on the catalytic activity decay and the acid and surface properties of the zeolite. The model considered in the present work has been applied to simultaneously correlate the TG data of the successive reaction cycles, the NH3 TPD experimental curves and the nitrogen adsorption isotherms as a function of the amount of coke in the catalyst. Very good correlation results have been obtained, proving that the method suggested allows a quantitative and simultaneous correlation of the global deactivation behaviour of the HZSM-5 in the catalytic cracking of LDPE. (c) 2007 Elsevier B.V. All rights reserved.