A study has been carried out on the effect of operating conditions (temperature and catalyst/feed ratio) on the yields of products and on the compositions of the lumps of gases and gasoline in the cracking of a mixture consisting of heavy coker naphtha and vacuum gas-oil (20 wt % in naphtha). The runs have been carried out in a MAT (microactivity test) reactor (500-550 degrees C; C/O = 4-8). The results, obtained for an equilibrated commercial catalyst in a refinery fluid catalytic cracking (FCC) unit, have been compared with those corresponding to the gas-oil feed. The presence of the naphtha in the feed has an inhibiting effect on the cracking of gas-oil and on the overcracking of the gasoline. This effect is avoided by using a sufficiently high catalyst/feed ratio (C/O > 6), and under these conditions, the yields of products and the composition of the gasoline are similar to those corresponding to the cracking of gas-oil, although the gasoline is more aromatic. The presence of HZSM-5 zeolite in the catalyst causes a significant increase in the amount of LPG (liquefied petroleum gases) (especially propene and i-butene) and efficiently contributes to decreasing the aromaticity of the gasoline in the cracking of the mixture. Furthermore, the HZSM-5 zeolite is efficient in increasing the concentration of olefins in the C-5 and C-6 fractions of the gasoline obtained by cracking the mixture.