The kinetic behavior has been studied for cracking a commercial feedstock on a hybrid catalyst (with 5 wt % of HZSM-5 zeolite) made up of 80 wt % of a catalyst equilibrated in a fluid catalytic cracking (FCC) unit and of 20 wt % of a catalyst prepared based on a HZSM-5 zeolite. A laboratory-scale reactor which allows for operation under conditions similar to those of commercial FCC units has been used. From the analysis of the gases and gasoline obtained in the 500-550 degrees C range, with contact times between 2 and 10 s and for a catalyst/feedstock = 6, the influence of HZSM-5 zeolite presence in the catalyst on gas and gasoline composition and on gasoline research octane number has been analyzed.