Data of the present work show that, in order to obtain high yields in light olefins, the ZSM-5 zeolite - the cracking component of the hybrid catalyst, must have a relatively low SiO2/Al2O3 mol ratio, so that its density of acid sites is high, thus resulting in a high total conversion. Such zeolite must also exhibit relatively mild acidity strength, favouring thus a high propylene/ethylene product ratio. On the other hand, these milder acid sites also lead to a lower coke deposition and thus to a greater on-stream catalyst stability because the cleaning action of the hydrogen spilt-over species is much eased. Feed contamination by trimethylbenzenes is an excellent tool for providing further evidence of the coke cleaning effect by the hydrogen spilt-over species, the latter being generated by the Ni supported co-catalyst. In the case of blending of petroleum hydrocarbon feedstocks with non-oil derived compounds, the latter should not have strong competitive adsorption properties such as those found in polymethylbenzenes (ex: 1,3,5-trimethylbenzene), otherwise rapid fouling phenomena may occur. (C) 2009 Elsevier B.V. All rights reserved.