The catalytic degradation of both low-and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) has been investigated using MCM-41, a mesoporous aluminosilicate recently discovered, as catalyst. The results obtained have been compared to those of ZSM-5 zeolite and amorphous silica-alumina. For all the studied plastics, MCM-41 has been found more active than the amorphous SiO2-Al2O3, asa consequence of the higher surface area and the uniform mesoporosity present in the former. Compared to ZSM-5, MCM-41 exhibits a lower activity for the degradation of linear and low branched polymers (HDPE and LDPE, respectively), which can be related to the higher strength of the zeolite acid sites. However, the opposite is observed for the cracking of highly substituted plastics such as PP due to the severe steric hindrances these molecules encounter to enter into the narrow pores of the zeolite, as confirmed by molecular simulation measurements. Moreover, for the cracking of LDPE, HDPE, and PP, the selectivities toward hydrocarbons in the range of gasolines and middle distillates obtained over MCM-41 are clearly higher than those of ZSM-5. Therefore, MCM-41 is a catalyst potentially interesting for the conversion of polyolefinic plastic wastes into liquid fuels.