A commingled polyethylene/polypropylene (PE/PP) mixture was pyrolysed over used fluid catalytic cracking (FCC) commercial equilibrium catalyst (RCat-cl) using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The conversion at 390 degrees C for RCat-cl catalyst (81.5 wt%) gave much higher yield than silicate (only 18.3 wt%). Greater product selectivity was observed with RCat-cI as a post-use catalyst with about 53 wt% olefins products in the C-3-C-6 range. The selectivity could be further influenced by changes in reaction conditions. Valuable hydrocarbons of olefins and iso-olefins were produced by low temperatures and short contact times used in this study. It is demonstrated that the use of spent FCC commercial catalyst and under appropriate reaction conditions can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products. (c) 2007 Elsevier B.V. All rights reserved.