<p>Typical FCC catalysts were modified with poly(siloxanes) (PSO) and the effects on catalyst properties and performances were evaluated. It was observed that the addition of PSO caused the consistent increase of the mechanical resistance and accessibility/attrition ratios of the obtained catalysts, leading to attrition indexes that were significantly lower than the reference values of catalysts prepared in absence of this additive. The accessibility values after deactivation of the modified catalysts were also higher than the ones obtained for the reference formulation prepared in absence of PSO. For catalysts treated with PSO, the acidity measured by n-propylamine adsorption was slightly higher than observed for the reference formulation. Besides, catalyst activity (measured as the gasoil cracking in a fluidized bed ACE laboratory unit) and selectivity to LPG + gasoline were also higher for the modified samples. Trying to explain these results, individual FCC ingredients and mixtures of ingredients were impregnated with PSO. Impregnated samples were characterized and catalytically tested in a fixed bed laboratory MAT unit using n-decane as feed. It was observed that poly(siloxanes) interacts with zeolite (USY), reducing the activity and increasing selectivity to light olefins. On alumina the effect of small amount of PSO was insignificant. For a mixture of USY, alumina and kaolin, the catalyst activity was roughly the same after treatment with PSO. It is concluded that the higher catalyst activity observed for gasoil cracking can be associated with the higher accessibility and higher total acidity of the modified catalysts.</p>