The effect of coke deposition on the product selectivity of FCC catalyst/ZSM-5 mixtures has been studied in the presence and absence of contaminant metals. In order to investigate the influence of coke deactivation on activity and selectivities experiments were also performed with coke-free and pre-coked FCC catalysts and FCC catalyst/ZSM-5 blends. The cracking reactions were conducted with a heavy vacuum gas-oil in a microactivity test unit at high temperature and short contact time. The effectiveness of ZSM-5 is mainly reflected by the selectivities to C-3 and C-4 products, especially olefins, and these selectivities revealed that the deactivation of ZSM-5 and FCC catalyst by coke was similar in the absence of metals. In the presence of metals ZSM-5 was less deactivated by coke formation than the FCC catalyst resulting in an enhanced effectiveness of ZSM-5 in both, the non-coked ZSM-5/FCC catalyst combinations and the pre-coked ZSM-5/FCC catalyst blends. The selectivities of C-6-C-9 olefins were not affected by pre-coking of the FCC catalyst prior to microactivity testing either in the metals-free or in the metallated state. C-6-C-9 olefins are reactants for ZSM-5 and therefore, the effects of coke deactivation on ZSM-5 performance cannot be ascribed to different olefin concentrations in the gasoline range. Coking of catalysts lowered the cracking activity of the FCC catalysts and therefore the reaction severity was boosted to achieve a given conversion and this was done by increasing the catalyst mass. Hence, the increasing effectiveness of metallated ZSM-5 with increasing coke-on-catalyst can be ascribed to both, the increase in FCC catalyst/ZSM-5 mass in the reactor at constant conversion and the lower deactivation rate of ZSM-5 by coke compared with the decay of the FCC catalyst.