Abstract The specific effects of varying the zeo-lite/matrix ratio in the fluid catalytic cracking (FCC) and the catalyst effects on activity and selectivity for light cycle oil (LCO) yield from the cracking of feedstocks of different physicoche-mical characteristics , were studied using a microactivity test (MAT) and two feedstocks, aliphatic and naphthe-nic. With a naphthenic feedstock, highest conversions, gasoline, LCO, dry gas and LPG yields are obtained by increasing zeolite/matrix ratio. Diminishment in slurry yield and in the quality of LCO due to the content of aromatics in LCO are also obtained. With aliphatic feedstock conversions, dry gas, LPG, gasoline and coke yields are improved; LCO and slurry yields are decreased with the increased use of zeolite in the catalyst. The quality of LCO is improved with aliphatic feedstocks; however, zeolite produced a negative effect on this quality. Key words Zeolites, catalysts, light cycle oil. Palavras chave Zeolitas, catalisadores, oleo leve de reciclo.