The performance of ZSM-5 as a fluid catalytic cracking (FCC) catalyst additive has been tested in a microactivity test unit for the cracking of gas oil. Laboratory-synthesized ZSM-5 samples, with various Si/Al ratios, fresh or hydrothermally dealuminated were tested. A 2 wt % ZSM-5, on total catalyst weight, found by previous investigators to be the optimum additive concentration, was used for all of the experiments. It has been found that a direct and smooth correlation exists between the product yields and the total number of ZSM-5 acid sites measured by ammonia temperature-programmed desorption tests. This is a unique function of the aluminum content of fresh samples as well as the temperature of the hydrothermal deactivation of steamed ZSM-5 zeolite samples. Previous works have been qualitatively compared to the results of this work and have been classified on the basis of different total acidity regions of the ZSM-5 used in each case. Higher total acidity results in gasoline loss, liquified petroleum gases (LPG) and ethylene increases, and research octane number gain. The C-5 aliphatics are increased in the low acidity region because of cracking and isomerization of larger alkenes, while an increase of the total ZSM-5 acidity resulted in a monotonic decrease of all of the gasoline range hydrocarbons, except of n-C-5 alkane, C-6-C-7 aromatics, and C-5 naphthenes. The branched/linear (B/L) ratios for C-5-C-6 alkenes were found to increase with acidity, while the B/L ratios for C-5-C-9 alkanes decrease. The number and strength of the additive＇s acid sites control the contribution of the different reaction paths, through which cracking, isomerization, and aromatization occur, to the final product distribution. The sizes of the ZSM-5 particles have a strong effect on the changes in product yields and gas and gasoline compositions. Smaller particles favor a decrease in gasoline and an increase in LPG yield more than larger particles. The effect is more pronounced in the high acidity region where the decrease in the yields of C7+ isoalkanes, naphthenes, and aromatics is favored by small particle additives, while the effect of particle size on gasoline range hydrocarbons is clearly evident on the yields of gaseous products.