The effect of the addition of a nanocrystalline H-beta zeolite in the pyrolysis of a commercial LDPE has been studied using dynamic conditions in a thermobalance and in a batch reactor, the results have been compared with those obtained using a HZSM-5 zeolite. The H-beta catalyst has an important influence on the decomposition of the polymer even at very low zeolite concentration. Although this mixture has a saturating effect at a certain concentration of catalyst (around 7%), a remarkable effect in the first steps of LDPE decomposition as increasing the catalyst loading has been observed. This important catalytic effect has been attributed to the external acid sites present in the H-beta zeolite. The study of the gaseous products evolved in the catalytic pyrolysis shows the influence of the structure and the temperature on the composition of the gas, and a possible effect of the deactivation of the catalyst. The isobutane/n-butane ratio obtained showed an important variation with temperature and was higher in the gas produced using H-beta than with HZSM-5. This different behaviour can be explained by the different pore structure. A preliminary study of the coke formed with the H-beta zeolite has been carried out and the results show that the zeolite causes an important delay in the oxidation process.