The effects of high severity operation and ZSM-5 addition on the FCC product yield structure have been studied in a MAT unit. VGO cracking at high reaction temperatures of 500-650 degrees C increases the yield of light olefins (propylene and butenes) with a corresponding loss in gasoline yield and increase in dry gas formation. Similar behavior is observed with the addition of 0-20 wt.% ZSM-5 additive, however, with no increase in dry gas. The combination of the two effects (high severity and ZSM-5 addition) makes the FCC unit an excellent source of light olefins for downstream petrochemical and alkylation units. A novel downer reactor FCC configuration is proposed to suppress thermal cracking reactions and dry gas formation during high severity operation.