It has been found that the cracking activity of MCM-41 for a reaction catalyzed by strong acids site, such as n-heptane cracking, is much lower than that of a USY zeolite, and similar to that of amorphous silica-alumina. The higher activity of USY is due to the presence of stronger Bronsted acid sites in the zeolite. In the case of gas oil cracking, the greater accessibility of the large molecules to acid sites in MCM-41 relative to USY makes the difference in activity between these two catalysts much smaller than for the pure hydrocarbon. In the calcined state MCM-41 is more active and gives more gasoline and less gases and coke than amorphous silica-alumina. However, when the catalysts were steamed, the pores of MCM-41 collapsed and its surface area and gas oil cracking was strongly reduced, resulting in a much less active catalyst than the steamed amorphous silica-alumina. An increase in the pore walls diameter from 7 to 11 Angstrom did not solve the hydrothermal stability limitations of the MCM-41. Samples with higher Al content showed a stronger surface area reduction upon steaming.