A mesoporous HMCM-41 type material and a nanosized HZSM-5 (n-HZSM-5) zeolite have been investigated as catalysts for the conversion of a plastic mixture consisting of polypropylene and both low- and high-density polyethylenes. The effects of the plastic/catalyst mass ratio (PIC) and the temperature have been studied using a batch reactor. Both materials present a high activity for the conversion of the polyolefin mixture, leading to almost total plastic conversion as the temperature is increased from 375 to 450 degreesC or as the P/C ratio is varied from 200 to 4. The product distributions obtained with these two catalysts are completely different, which is related to the prevailing cracking mechanism. n-HZSM-5 zeolite, with high external surface area and strong acid sites, promotes end-chain scission reactions of the polymers, leading to light hydrocarbons, with around 80-90% of the products in the range C-3-C-6. In contrast, heavier products (C-5-C-12 and C-13-C-22) are obtained over HMCM-41, indicating that random scission reactions are predominant as a result of the large pores and mild acidity of this material. Changes in the reaction temperature and/or the P/C ratio over the HMCM-41 catalyst allowed for optimization of the selectivity toward gasoline and middle distillate fractions up to values close to 90%.