We analyzed the effect of the addition of a Lewis acid (AlCl3) or a Lewis base (EB-ethyl benzoate) to the supported system rac-EtInd(2)ZrCl(2)/MAO/SiO2. We studied the additive effect in two different ways: as external and internal component. The results indicate that the additives are acting in different manners depending on the method of addition and the type of additive. An important result was obtained when the Lewis acid was added as an internal component. The molar ratio [MAO]: [AlCl3] of the co-impregnation step would be the most important factor to obtain an active supported catalyst.