The deactivation process of HZSM5 zeolite during the catalytic degradation of LDPE and HDPE has been studied by repeated batch operation using the same sample of catalyst in consecutive cycles until almost a total loss in the activity of the catalyst was observed, The gaseous and condensed products generated in each cycle were analysed as well as the coked catalyst obtained. They were characterized by nitrogen adsorption-desorption isotherms at 77 K, temperature programmed desorption of ammonia and thermogravimetric analysis. The deactivation process of HZSM5 depends on the type of polyethylene used. With LDPE the zeolite showed a slow deactivation. In the first four consecutive cycles the coke deposit increased, significantly changing the properties of the catalyst although the yield of gaseous and condensed products remained nearly constant throughout. From the fifth cycle on no more coke was deposited on the catalyst but the composition of products changed progressively, tending to that obtained in a thermal process. This behaviour could be related to the change in coke composition because of the effect of the temperature and the time. On the other hand, with HDPE, a clear relationship of the coke content, acidity, activity and textural properties of the catalyst with the products generated was observed. (c) 2007 Elsevier B.V. All rights reserved.