Gasoline samples resulting from riser cracking with catalyst mixtures consisting of RE-USY blended with a commercial ZSM-5 at additive levels up to 75 wt% were analyzed to give the carbon number distribution of paraffinic, olefinic, naphthenic and aromatic components using PIONA, PONA and an open tubular GC system. We observed that the advantages of operating with unusually large amounts of ZSM-5 additive also showed up in other cracked products beside C-3 and C-4 olefins such as the significant improvement in isoamylene yield (mainly 2-methyl-1-butene and 2-methyl-2-butene) in the gasoline pool. A catalyst system containing 25 wt% ZSM-5 additive blended with 75 wt% steamed RE-USY provided over 30% increase in isoamylene yield in the gasoline pool (measured on feed basis) compared with the base catalyst at equivalent conversion. Overall, C-5 olefin content remained unchanged but selectively shifted to iC(5) olefins at the expense of nC(5)(=). Gasoline range olefins (C-6(+)) decreased. In addition, aromatics content (both benzene and alkylbenzenes) in the gasoline fraction (normalized to a constant feed basis) remained fairly constant or slightly lower. The increase in aromatics generally observed (on gasoline basis) is due to concentration effects. Both PIONA and PONA data indicate gasoline volume loss occurs primarily from a decrease in isoparaffins, iso-olefins, n-olefins and n-paraffins in the C-6-C-9 range. The level of ring paraffins (or naphthenes), ring olefins (or unsaturated naphthenes), and indans and tetralins remains essentially constant. PONA data of gasoline samples also indicate that the gasoline loss (on feed basis) occurs in the following order : paraffins>>mono-olefins>>saturated naphthenes; the unsaturated naphthenes, dicycloparaffins, di- and cyclo-olefins, and indans and tetralins remain essentially constant.