The effect of the rare-earth element (REE) content in dual-zeolite cracking catalysts on the conversion ofn-dodecane and ann-dodecane-2-methylthiophene mixture (5000 ppm S) has been studied. The catalysts have been synthesized using zeolites Y and ZSM-5 modified with REEs and phosphorus, respectively, as the active component. It has been shown that the presence of REE oxides in zeolite Y (at a constant SiO2/Al(2)O(3)ratio) affects the catalytic activity of cracking catalysts based on this zeolite. The dependence of the light olefin yield on the REE oxide content in zeolite Y passes through a maximum at 2-3 wt %. The presence of a sulfur compound in an amount of 5000 ppm (in terms of sulfur) in the feedstock leads to a decrease in the model hydrocarbon conversion and the C-2-C(4)olefin yield. In the presence of a sulfur compound, the change in the C-2-C(4)olefin yield as a function of REE oxide content in zeolite Y is less pronounced than that in the case of individualn-dodecane cracking.