The influence of H-ZSM-5 addition to a HY catalyst in the cracking of a Gippsland reduced crude was investigated. The mode of pretreatment (particularly steaming) of the H-ZSM-5 additive appears to effect the reaction mechanism associated with octane enhancement. Cracking the feedstock over combinations of HY and unsteamed H-ZSM-5, suggest that the pentasil is active in the selective removal of linear alkanes from the heavier product fractions formed on HY, producing gasoline range linear alkanes. This results in a decreased branched to linear ratio for gasoline range alkanes. The corresponding ratio for alkenes is increased upon pentasil addition due to the isomerisation of linear alkenes. For combinations of HY with steamed H-ZSM-5, selective removal of alkanes is negligible. The major influence of the additive is to isomerise and crack linear alkenes, leading to increased branched to linear ratios for both alkanes and alkenes.