The trend in the FCC industry is towards increasing the rare-earth content of REUSY catalysts. Such catalysts provide higher acid site density than at lower rare-earth concentrations, which enhances both cracking rate and hydrogen-transfer activities. The beneficial effects of increasing hydrogen-transfer activity are the reduction in gasoline olefinicity and increasing gasoline yields. Increasing unit-cell size also has the undesired effect of reducing C-3-C-5 olefins. In practical applications this trade-off in the smaller olefins is often corrected by the use of ZSM-5 containing additives. In order to quantify the effects of varying cracking and hydrogen-transfer activity of the base FCC catalyst on ZSM-5 performance, REUSY catalysts with different rare-earth contents were blended with a ZSM-5 containing additive. These combinations were investigated in a microactivity test unit at short contact-time, high temperature and with a heavy vacuum gas-oil. Earlier studies have shown that the effectiveness of ZSM-5 was decreased by increasing the rare-earth content of the base catalyst. Increasing the rare-earth content had two effects: enhanced cracking activity and lowered gasoline olefinicity, A detailed analysis of the data revealed that both of these effects tend to reduce ZSM-5 effectiveness. In this work, the investigation of the effects of lower gasoline reactivity and higher cracking activity were accomplished by experiments with feeds of different crackability and yielding different olefin yields in the gasoline fraction. The following shifts were observed in the product distribution: the ratios of propene-to-i-butene increased significantly with the degree of depletion of gasoline range olefins by the different REUSY catalysts. The study shows that ZSM-5 addition enhanced isomerisation: i-butene and i-pentene formation was shifted by ZSM-5 towards thermodynamic equilibrium. The reduction of gasoline olefinicity by ZSM-5 was more pronounced in base catalysts of low hydrogen-transfer activity. The experimental settings used in this work have been shown to produce data being relatively close to those of riser pilot plants and therefore, this investigation also provides some guidance with regard to the commercial application of ZSM-5 additives.