A study has been made of the cracking of polyethylene (PE) and polypropylene (PP) (which are the main components of post-consumer plastic wastes) dissolved in the Light Cycle Oil (LCO) product stream of a commercial Fluid Catalytic Cracking (FCC) unit. The cracking has been carried out on a mesoporous silica (pore size between 3 and 30 nm) in the 723-823 K range. This strategy for upgrading plastics and solvents together avoids heat transfer limitations and other problems inherent to the cracking of solid plastics. The polyolefins are transformed mainly into the components that make up the pool of gasoline (C-5-C-12). Furthermore, the incorporation of polyolefins has a synergistic effect on the cracking of LCO and causes a major decrease in the content of aromatics of the pool of gasoline and an increase in the content of olefins, paraffins and i-paraffins. (C) 2003 Elsevier B.V. All rights reserved.