The role of polymeric structures and the presence of a catalyst in the pyrolysis of polyolefin mixtures and used tires was studied in a fixed bed reactor at 450 degrees C. Our results showed that while thermal pyrolysis of high density polyethylene (HDPE) produced 23.3%wt of condensable products, a mixture of polyolefins (HDPE, LDPE, and PP) showed an increase of more than 23%wt in this fraction. In both cases, a wide carbon number distribution was obtained for the condensable products, confirming that there was not any selectivity. By using HZSM-5 zeolites, the pyrolysis of pure HDPE was dramatically changed with both more gaseous products and a major selectivity toward C10-C16 hydrocarbons in the condensable fraction. It is noteworthy that, although the PP present in the mixture increases the production of lighter hydrocarbons as compared with pure HDPE, no major differences in product distribution were observed between HDPE and polyolefin mixture in catalytic pyrolysis. However, the zeolites used improved the quality of condensable products that can be used as potential fuels or feedstocks. When used tires were thermally pyrolyzed, a decrease of 33.5%wt in the solid product fraction (char and unreacted polymer) compared with pure HDPE was observed. In the case of catalytic pyrolysis of used tires, the increase of gaseous products is not as pronounced as in pure HDPE. Moreover, the carbon number distribution under catalytic pyrolysis was significantly different compared to that obtained for polyolefins. These results give evidence about the strong effect of both polymer microstructure and mixture on the pyrolysis processes.