FCC yields were quantified for operation under conditions that maximize the production of light olefins. These conditions include high reaction temperatures, use of low unit cell size base catalysts (USY and RE-USY), and addition of large amounts of ZSM-5. Adding 25% ZSM-5 additive nearly tripled propene yields, and increased butenes by about 60%. The fraction of isobutene in the butenes rose from 32-35 wt.-% to its equilibrium value of about 40%. For the base catalysts alone, increasing the reaction temperature from 811 K to 852 K gave a modest increase in C-3-C-4 olefins, but this effect was largely absent for the ZSM-5/base catalyst mixture. This suggests that raising temperature loses effectiveness as a means of increasing light olefins yield, in the presence of large amounts of ZSM-5. Addition of 25% ZSM-5 additive had effects on C-2- species and butadiene which have not generally been observed with lower levels of ZSM-5. Ethylene yields increased, and methane and ethane yields decreased, while butadiene yields were halved. Increasing temperature gave significant increases in the yields of butadiene, all C-2- species, and a decrease in butanes; these trends are the opposite of those seen with ZSM-5 addition. PIONA analysis of selected gasoline samples showed that raising temperature gave a more olefinic gasoline. ZSM-5 addition increased olefin branching and shifted olefins and paraffins to lower carbon numbers. Most yield shifts could be understood in terms of how changing temperature and adding ZSM-5 affect the relative rates of cracking versus hydrogen transfer for the gasoline-range olefins.