Provided herein is a method and apparatus for selective deep catalytic cracking of petroleum naphthas or other hydrocarbon feedstocks. The feed comprising the hydrocarbons to be cracked and steam, in well-defined proportions, is first sent into a precatalytic zone (Zone I) of a cracking reactor, preferably a tubular reactor. In a preferred embodiment, Zone I, contains beads of some catalytically mildly active porous material such as quartz or quartz doped with Cr--Al, is set at a temperature T.sub.1. The gas steam flows then through a catalyst bed, called Zone II, where the catalytic reaction takes place. Zone II is set at temperature T.sub.2 and contains a zeolite type catalyst, preferably ZSM5 zeolite or, most preferably, a hybrid zeolite catalyst. It is shown that by moderately increasing the temperature T.sub.1 of Zone I, it is possible to have an increased total conversion of n-hexane used as model molecule for petroleum naphthas and an increased yield of light olefins (and aromatics), when compared to the parent ZSM-5 zeolite catalyst The ethylene/propylene product ratio is closely dependent of the temperature T.sub.1, if all other reaction parameters are kept constant and can be widely varied (from 1.0 to 2.0 for instance) for a limited range of variation of temperature T.sub.1.