The cracking behaviour of three medium-pore zeolites -SVR, ITH, and MFI, with varying Si/Al molar ratio, was evaluated using an equilibrium FCC catalyst (E-Cat) and a hydrotreated vacuum gasoil (VGO) in a microactivity test unit (MAT). The increase in the yield of light olefins (21-29 wt% over E-Cat/additive vs. 16 wt% over E-Cat) was accompanied by a drop in the yield of gasoline (26-39 wt% over E-Cat/additive vs. 43 wt% over E-Cat) due to the cracking of reactive species in gasoline fraction (mainly olefins and isoparaffins). With increasing Si/Al ratio in zeolites MFI-280 and -SVR-120, propylene yield passed through maxima reaching 13.5 wt% and 12.8 wt%, respectively, compared with 7.0 wt% over E-Cat. Maximum propylene yield occurred partially due to gasoline over-cracking to light olefins (inverse relation with Si/Al ratio). The effect of MR crystal size showed that large size crystals enhanced the yield of light olefins as compared to small size crystals due to secondary cracking of gasoline molecules. Results of kinetic study based on 4-lump model showed proportional relation between MFI Si/Al ratio and activation energy for the conversion of gasoline to gases. Activation energy increased from 14.2 kcal/mol to 16.3 kcal/mol as Si/Al ratio increased from 30 to 2000. (C) 2017 Elsevier B.V. All rights reserved.