The addition of ZSM-5 to fluidized catalytic cracking (FCC) units can greatly increase yields of C-3-C-5 olefins, which are key intermediates in the manufacture of clean fuels such as ethers and alkylate. Understanding how ZSM-5 functions in catalytic cracking helps in optimally applying this zeolite, and can provide guidance in developing next-generation catalysts. Here, the results of model compound studies are presented, which elucidate the changes in reaction pathways that occur in the FCC unit upon ZSM-5 addition. The observed increase in C-5- and decrease in C7+ olefins and paraffins can be accounted for by ZSM-5＇s interception of heavy olefin intermediates that would otherwise undergo hydrogen transfer over the base faujasitic catalyst, Interpretations are also offered for the high C-3=/C-4=ratio seen for high activity ZSM-5 additives, and for the high gasoline selectivity associated with high Si/Al ZSM-5 additives. (C)2000 Elsevier Science B.V. All rights reserved.