The effect of the addition of high levels of ZSM-5 to steamed rare earth ultra-stabilized fluid catalytic cracking (FCC) catalyst has been studied in an isothermal, bench scale, once-through riser pilot plant. Catalyst mixtures consisting of RE-USY blended with ZSM-5 at additive levels of 12.5, 25, 50, and 75 wt.-% were examined. Results indicate yields of fight alkenes could be maximized by operating with large amounts of ZSM-5. For the particular gas oil used in this study, a maximum of light alkenes yield occurs at an additive level of about 25 wt.-%; other feeds can show a different response, At the 25 wt.-% additive level we observed maximum yield increases of about 150% in propene and isobutene, and 120% in n-butenes. At lower additive levels (<25 wt.-%), conversion of reactive gasoline-range species (mainly alkenes) is not complete. Above 25 wt.-%, conversion of these species is essentially complete, and the major effect of ZSM-5 is dilution of the base cracking catalyst with concomitant loss in overall conversion. The FCC naphtha octane (both RON and MON) also increased with additive level up to 25 wt.-%. We also observed an increase in ethylene yield, along with suppression of some undesirable products such as methane. In the presence of 25 wt.-% ZSM-5 additive, there were no significant shifts in bottoms (or HCO, heavy cycle oil) and light cycle oil (LCO) at constant conversion and only a slight decrease in coke yield at constant cat/oil ratio.